rebase -i: allow replaying commits with empty log messages

git-rebase--interactive handles empty log messages inconsistently
between enabled and disabled fast-forwards. By default, commits with
empty log messages are rebased successfully like in non-interactive
mode. In contrast, the `--no-ff` option aborts the replay of such
commits.

In line with not verifying rebased commits and behaving like
git-rebase for `pick` lines, use the `--allow-empty-message` option
to replay commits. Root commits are replayed by recreating them in
`do_pick` using git-commit and all other commits are replayed using
git-cherry-pick in `pick_one`. Apply the option, understood by both
git-commit and git-cherry-pick, at the respective sites.

In case of `reword` and `squash`, continue to abort the rebase if the
_resulting_ commit would have no commit message. The rationale behind
this default is that patches and their log messages should be
verified at least once. For unchanged commits this is assumed to have
happened according to the author's standards when she created the
commits for the first time. While the empty log message can always be
kept in place by editing and resuming the aborted rebase, a debatable
alternative could be to teach git-rebase--interactive the option
`--allow-empty-message` for disabling complaints about empty log
messages even in changed commits.

The `fixup` case is different again because it throws away the second
commit's log message and uses the first log message for the changed
commit. Do not abort the rebase if that message is empty either since
it is assumed to have been verified already.

The remaining to-do list command `edit` is handled just like `pick`
for this matter, because git-rebase--interactive replays the named
commit without changes before the rebase is interrupted and the user
can make her changes to the replayed commit.

Add tests. In particular, design the `squash`-specific test case such
that it involves interim commits and `fixup` steps. Interim commits
should not trigger failures themselves and `fixup` steps should not
let git-rebase--interactive forget that it is still dealing with a
`squash` result.
rebase -i: allow squashing empty commits without complaints

The to-do list commands `squash` and `fixup` apply the changes
introduced by the named commit to the tree but instead of creating
a new commit on top of the current head it replaces the previous
commit with a new commit that records the updated tree. If the
result is an empty commit git-rebase stops with the error message

   You asked to amend the most recent commit, but doing so would make
   it empty. You can repeat your command with --allow-empty, or you can
   remove the commit entirely with "git reset HEAD^".

This message is not very helpful because neither does git-rebase
support an option `--allow-empty` nor does the messages say how to
resume the rebase. Firstly, change the error message to

   The squash result is empty and --keep-empty was not specified.

   You can remove the squash commit now with

     git reset HEAD^

   Once you are down, run

     git rebase --continue

If the user wishes to squash a sequence of commits into one
commit, f. i.

   pick A
   squash Revert "A"
   squash A'

, it does not matter for the end result that the first squash
result, or any sub-sequence in general, is going to be empty. The
squash message is not affected at all by which commits are created
and only the commit created by the last line in the sequence will
end up in the final history. Secondly, print the error message
only if the whole squash sequence produced an empty commit.

Lastly, since an empty squash commit is not a failure to rewrite
the history as planned, issue the message above as a mere warning
and interrupt the rebase with the return value zero. The
interruption should be considered as a notification with the
chance to undo it on the spot. Specifying the `--keep-empty`
option tells git-rebase to keep empty squash commits in the
rebased history without notification.

Add tests.
rebase -i: allow rewording empty commits without complaints

The to-do list command `reword` replays a commit like `pick` but lets
the user also edit the commit's log message. If `--keep-empty` is
passed as option to git-rebase--interactive, empty commits ought to
be replayed without complaints. However, if the users chooses to
reword an empty commit by changing the respective to-do list entry
from

    pick fa1afe1 Empty commit

to

    reword fa1afe1 Empty commit

, then git-rebase--interactive suddenly fails to replay the empty
commit. This is especially counterintuitive because `reword` is
thought of as a `pick` that alters the log message in some way but
nothing more and the unchanged to-do list entry would not fail.

Handle `reword` by cherry-picking the named commit and editing the
log message using

    git commit --allow-empty --amend

instead of

    git commit --amend.

Add test.
rebase -i: hide interactive command messages in verbose mode

git-rebase--interactive prints summary messages of the commits it
creates in the final history only if the `--verbose` option is
specified by the user and suppresses them otherwise. This behaviour
is implemented by wrapping git-commit calls in a shell function named
`output` which redirects stderr to stdout, captures stdout in a shell
variable and ignores its contents unless the command exits with an
error status.

The command lines used to implement the to-do list commands `reword`
and `squash` print diagnostic messages even in non-verbose mode. The
reason for this inconsistency is that both commands launch the log
message editor which usually requires a working terminal attached to
stdin. Temporarily redirect the editor output to a third file
descriptor in order to ship it around the capture stream. Wrap the
respective git-commit command lines in `output`.

fake_editor prints the to-do list before and after applying the
`FAKE_LINES` rewrite rules to it. Redirect this debug output to
stderr so that it does not interfere with the git-rebase status
output. Add test.
rebase -i: failed reword prints redundant error message

The to-do list command `reword` replays a commit like `pick` but lets
the user also edit the commit's log message. If the edited log
message is empty or is found ill-formatted by one of the commit
hooks, git-rebase--interactive prints three error messages to the
console.

    1. The git-commit output, which contains all the output from hook
       scripts.
    2. A rebase diagnosis saying at which task on the to-do list it
       got stuck.
    3. Generic presumptions about what could have triggered the
       error.

The third message contains redundant information and does not add any
enlightenment either, which makes the output unnecessarily longish
and different from the other command's output. For instance, this is
what the output looks like if the log message is empty (contains
duplicate Signed-off-by lines).

    (1.) Aborting commit due to empty commit message. (Duplicate Signed-off-by lines.)
    (2.) Could not amend commit after successfully picking fa1afe1... Some change
    (3.) This is most likely due to an empty commit message, or the pre-commit hook
         failed. If the pre-commit hook failed, you may need to resolve the issue before
         you are able to reword the commit.

Discard the third message.

It is true that a failed hook script might not output any diagnosis
but then the generic message is not of much help either. Since this
lack of information affects the built-in git commands for commit,
merge and cherry-pick first, the solution would be to keep track of
the failed hooks in their output so that the user knows which of her
hooks require improvement.
commit: allow disabling pre-commit and commit-msg separately

Introduce the git-commit command line options `--no-pre-commit` and
`--no-commit-msg` to disable the pre-commit and commit-msg hooks,
respectively. Make `--no-verify` a synonym for specifying both at the
same time.

This change is motivated by an internal usage of git-commit in
git-rebase--interactive to disable pre-commit while keeping
commit-msg enabled when rewording a commit.

Make `test_commit` forward unknown options to git-commit instead of
teaching it all possible options. In order to support leading double
dashes in `<message>`, stop interpreting `test_commit` arguments
following a `--` argument as options. This wasn't a problem before
because the first unknown option would be used as `<message>`.

Allow disabling tag creation to avoid name clashes when using
`test_commit` with the same arguments several times from the same
test suite. By default, `test_commit` tags successful commits using
git-tag for easy reference. The `--notag` option skips this step.

Add tests.
rebase -i: squash skips commit-msg hook

Using the to-do list command `squash` the user can specify two or
more commits and git-rebase creates one commit that introduces all
their changes combined. The authorship for the created commit is
taken from the first commit specified and the user can edit the log
message. There is a variant of `squash` available named `fixup` which
also takes the first log message without asking for user input.

While it is reasonable to not verify replayed changes twice or
rejecting some other author's changes in her name, it is insufficient
to not verify the user input used as log message in the case of
`squash`. Specify the git-commit option `--commit-msg` when
committing the squash result to execute the commit-msg hook and
verify the new log message. For the same reasons the pre-commit hook
is disabled in all replay modes, the commit-msg hook gets disabled in
`fixup` mode.

Add tests. In addition to the existing test checking that the
pre-commit hook is disabled when simply picking a commit, provide a
test checking that the commit-msg hook is disabled as well.
rebase -i: reword executes pre-commit hook on interim commit

The to-do list command `reword` replays a commit like `pick` but lets
the user also edit the commit's log message. This happens in two
steps. Firstly, the named commit is cherry-picked. Secondly, the
commit created in the first step is amended using an unchanged index
to edit the log message. The pre-commit hook is meant to verify the
changes introduced by a commit (for instance, catching inserted
trailing white space). Since the committed tree is not changed
between the two steps and we do not verify rebased patches, do not
execute the pre-commit hook in the second step.

Specify the git-commit option `--no-pre-commit` to disable the
pre-commit hook when editing the log message. The commit-msg hook
will still be executed to verify the edited commit log message. As
before, if the hook finds the new log message ill-formatted, the
rebase will be interrupted with the unchanged commit replayed and the
new log message in `$GIT_DIR/COMMIT_EDITMSG`.

Add tests.
rebase -i: teach do_pick the option --edit

`do_pick` is the git-cherry-pick wrapper in git-rebase--interactive
that is used to implement the to-do list command `pick`. To cater for
the different pick behaviours (like `reword`), `do_pick` accepts
several options not only from the git-cherry-pick but also the
git-commit interface. Add the common option `--edit` to let the user
edit the log message of the named commit.

Loop over `$@` to parse the `do_pick` arguments. Assign the local
variable `edit` if one of the options is `--edit` so that the
remainder of `do_pick` can easily check whether the client code asked
to edit the commit message. If one of the options is unknown, mention
it on the console and `die`. Break the loop on the first non-option
and do some sanity checking to ensure that there exactly two
non-options, which are interpreted by the remainder as `<commit>` and
`<title>` like before.

`do_pick` ought to act as a wrapper around `cherry-pick`.
Unfortunately, it cannot just forward `--edit` to the `cherry-pick`
command line. The assembled command line is executed within a command
substitution for controlling the verbosity of `rebase--interactive`.
Passing `--edit` would either hang the terminal or clutter the
substituted command output with control sequences. Execute the
`reword` code from `do_next` instead if the option `--edit` is
specified.
rebase -i: implement reword in terms of do_pick

The to-do list command `reword` replays a commit like `pick` but lets
the user also edit the commit's log message. If one thinks of `pick`
entries as scheduled `cherry-pick` command lines, then `reword`
becomes an alias for the command line `cherry-pick --edit`. The
porcelain `rebase--interactive` defines a function `do_pick` for
processing the `pick` entries on to-do lists. Reimplement `reword` in
terms of `do_pick --edit`.

If the user picks a commit using the to-do list line

    reword fa1afe1 Some change

execute the command `do_pick --edit fa1afe1 "Some change"` which
carries out exactly the same steps as the case arm for `reword` in
`do_next` so far.
rebase -i: log the replay of root commits

The command line used to recreate root commits specifies the option
`-q` which suppresses the commit summary message. However,
git-rebase--interactive tends to tell the user about the commits it
creates in the final history, if she wishes (cf. command line option
`--verbose`). The code parts handling non-root commits and squash
commits all output commit summary messages. Do not make the replay of
root commits an exception. Remove the option to make the report of
the rebased history complete. Do not forget to wrap the git-commit
command line in `output` so that the summary is shown if git-rebase
is called with the `--verbose` option but suppressed otherwise.

It is OK that the commit summary is still suppressed when git-commit
is used to initialize the authorship of the sentinel commit because
this additional commit is an implementation detail hidden from the
final history. The removed `-q` option was probably introduced as a
copy-and-paste error stemming from that part of the root commit
handling code.
rebase -i: root commits are replayed with an unnecessary option

The command line used to recreate root commits uses `-C` to suppress
the log message editor. This is unnecessarily confusing, though,
because that suppression is a secondary effect of the option. The
main purpose of `-C` is to pull the metadata from another commit, but
here we know that this is a noop, since we are amending a commit just
created from the same data.

At the time `-C` was introduced, git-commit did not yet have a
documented `--no-edit`, and this was a reasonable way to get the
desired behavior. Switch it to use `--no-edit` to make the intended
effect more obvious.
rebase -i: commit only once when rewriting picks

The options passed to `do_pick` determine whether the picked commit
will be rewritten or not. If the commit gets rewritten, because the
user requested to edit the commit message for instance, let
`pick_one` merely apply the changes introduced by the commit and do
not commit the resulting tree yet. If the commit is replayed as is,
leave it to `pick_one` to recreate the commit (possibly by
fast-forwarding the head). This makes it easier to combine git-commit
options like `--edit` and `--amend` in `do_pick` because
git-cherry-pick does not support `--amend`.

In the case of `--edit`, do not `exit_with_patch` but assign
`rewrite` to pick the changes with `-n`. If the pick conflicts, no
commit is created which we would have to amend when continuing the
rebase. To complete the pick after the conflicts are resolved the
user just resumes with `git rebase --continue`.

git-commit lets the user edit the commit log message by default. We
do not want that for the rewriting git-commit command line because
the default behaviour of git-rebase is exactly the opposite. Pass
`--no-edit` when rewriting a picked commit. An explicit `--edit`
passed to `do_pick` (for instance, when reword is executed) enables
the editor launch again. Similarly, pass `--allow-empty-message`
unless the log message is edited.

If `rebase--interactive` is used to rebase a complete branch onto
some head, `rebase` creates a sentinel commit that requires special
treatment by `do_pick`. Do not finalize the pick here either because
its commit message can be altered as for any other pick. Since the
orphaned root commit gets a temporary parent, it is always rewritten.
Safely use the rewrite infrastructure of `do_pick` to create the
final commit.
rebase -i: do not die in do_pick

Since `do_pick` might be executed in a sub-shell (a modified author
environment for instance), calling `die` in `do_pick` has no effect
but exiting the sub-shell with a failure exit status. The
git-rebase--interactive script is not terminated. Moreover, if
`do_pick` is called while a squash or fixup is in effect,
`die_with_patch` will discard `$squash_msg` as commit message.
Lastly, after a `die` in `do_pick` `do_next` has no chance to
reschedule tasks that failed before changes could be applied.

Indicate an error in `do_pick` using return statements and properly
kill the script at the call sites. Although possible in principle,
the issued error messages are no more indicating whether `do_pick`
failed while applying or while committing the changes. This reduces
code complexity at the call site and does not matter from a user's
point of view because a glance at the index reveals whether there are
conflicts or not and in-depth troubleshooting is still possible using
the `--verbose` option.

Remove the commit message title argument from `do_pick`'s interface,
which has become unused.
rebase -i: teach do_pick the option --amend

`do_pick` is the git-cherry-pick wrapper in git-rebase--interactive
that is used to implement the to-do list commands `pick`, `reword`
and `edit`. To cater for the different pick behaviours (like
`squash`), `do_pick` accepts several options not only from the
git-cherry-pick but also the git-commit interface.

Add the option `--amend` from the git-commit interface to the options
pool of `do_pick`. It creates a new commit for the changes introduced
by the picked commit and the previous one. The previous commit is
then replaced with the new commit. If no other options are specified,
the log message of the previous commit is used.

Be careful when `--amend` is used to pick a root commit because HEAD
might point to the sentinel commit but there is still nothing to
amend. Be sure to initialize `amend` so that commits are squashed
even when git-rebase--interactive is interrupted for resolving
conflicts. It is not a mistake to do the initialization regardless of
any conflicts because `amend` is always cleared before the next to-do
item is processed.
rebase -i: teach do_pick the option --file

`do_pick` is the git-cherry-pick wrapper in git-rebase--interactive
that is used to implement the to-do list command `pick`, `reword` and
`edit`. To cater for the different pick behaviours (like `squash`),
`do_pick` accepts several options not only from the git-cherry-pick
but also the git-commit interface.

Add the option `--file` from the git-commit interface to the options
pool of `do_pick`. It expects an argument itself which is interpreted
as a file path and takes the commit message from the given file. If
`--file` is passed to `do_pick`, assign the given file path to the
local variable `rewrite_message` and relay the option

    --file "$rewrite_message"

to the git-commit command line which creates the commit.
rebase -i: prepare for squash in terms of do_pick --amend

Rewrite `squash` and `fixup` handling in `do_next` using the sequence

    pick_one
    commit

in order to test the correctness of a single `do_squash` or
parameterised `do_pick` and make the subsequent patch reimplementing
`squash` in terms of such a single function more readable.

Do not call `rm -f "$GIT_DIR"/MERGE_MSG` since it has no effect on
the state after git-rebase--interactive terminates. The option `-F`
makes git-commit ignore `MERGE_MSG` for the log message. If
git-commit succeeds, `MERGE_MSG` is removed, and if it fails,
`MERGE_MSG` is overwritten by the error sequence `die_failed_squash`.
In summary, removing `MERGE_MSG` neither influences the squash commit
message nor the file state after git-commit returns.

Moreover, `pick_one` ignores `$GIT_DIR/SQUASH_MSG` and does not touch
`$squash_msg` so that it is correct to execute `pick_one` immediately
before git-commit.

Might be squashed into the subsequent commit.
rebase -i: implement squash in terms of do_pick

The to-do list command `squash` and its close relative `fixup` replay
the changes of a commit like `pick` but do not recreate the commit.
Instead they replace the previous commit with a new commit that also
introduces the changes of the squashed commit. This is roughly like
cherry-picking without committing and using git-commit to amend the
previous commit.

The to-do list

    pick   a Some changes
    squash b Some more changes

gets translated into the sequence of git commands

    git cherry-pick a
    git cherry-pick -n b
    git commit --amend

and if git-cherry-pick supported `--amend` this would look even more
like the to-do list it is based on

    git cherry-pick a
    git cherry-pick --amend b.

Since `do_pick` takes care of `pick` entries and the above suggests
`squash` as an alias for `pick --amend`, reimplement `squash` in
terms of `do_pick --amend`. Introduce `$squash_msg` as the commit
message via the `--file` option. When the last commit of a squash
series is processed, the user is asked to review the log message.
Pass `--edit` as additional option in this case. The only difference
in the options passed to git-commit and `do_pick` is the omitted
`--no-verify`. However, `do_pick` does not execute the verification
hooks anyway because it solely replays commits and assumes that they
have been verified before.
rebase -i: explicitly distinguish replay commands and exec tasks

There are two kinds of to-do list commands available. One kind
replays a commit (`pick`, `reword`, `edit`, `squash` and `fixup` that
is) and the other executes a shell command (`exec`). We will call the
first kind replay commands.

The two kinds of tasks are scheduled using different line formats.
Replay commands expect a commit hash argument following the command
name and exec concatenates all arguments to assemble a command line.

Adhere to the distinction of formats by not trying to parse the
`sha1` field unless we are dealing with a replay command. Move the
replay command handling code to a new function `do_replay` which
assumes the first argument to be a commit hash and make no more such
assumptions in `do_next`.
rebase -i: parse to-do list command line options

Read in to-do list lines as

    command args

instead of

    command sha1 rest

so that to-do list command lines can specify additional arguments
apart from the commit hash and the log message title, which become
the non-options in `args`. Loop over `args`, put all options (an
argument beginning with a dash) in `opts`, stop the loop on the first
non-option and assign it to `sha1`. The loop does not know the
options it parses so that options that take an argument themselves
are not supported at the moment. Neither are options that contain
spaces because the shell expansion of `args` in `do_next` interprets
white space characters as argument separator, that is a command line
like

    pick --author "A U Thor" fa1afe1 Some change

is parsed as the pick command

    pick --author

and the commit hash

    "A

which obviously results in an unknown revision error. For the sake of
completeness, in the example above the message title variable `rest`
is assigned the string 'U Thor" fa1afe1 Some change' (without the
single quotes).

Print an error message for unknown or unsupported command line
options, which means an error for all specified options at the
moment. Cleanly break the `do_next` loop by assigning the special
value 'unknown' to the local variable `command`, which triggers the
unknown command case in `do_cmd`.

The to-do list is also parsed when the commit hashes are translated
between long and short format before and after the to-do list is
edited. Apply the same procedure as in `do_cmd` with the exception
that we only care about where the options stop and the commit hash
begins. Do not reject any options when transforming the commit
hashes.