squash! commit: allow disabling pre-commit and commit-msg separately

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.
squash! rebase -i: reword executes pre-commit hook on interim commit

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`.
edit! rebase -i: teach do_pick the option --edit

insert --commit-msg
fixup! rebase -i: commit only once when rewriting picks
squash! rebase -i: root commits are replayed with an unnecessary option

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.
squash! rebase -i: allow rewording empty commits without complaints

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.
squash! rebase -i: allow squashing empty commits without complaints

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.
fixup! rebase -i: allow replaying commits with empty log messages
fixup! rebase -i: teach do_pick the option --file
edit! rebase -i: allow squashing empty commits without complaints

insert --allow-empty