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.
rebase -i: Teach do_pick the option --reset-author

`do_pick` is the git-cherry-pick wrapper in git-rebase--interactive
that is used to implement many of the to-do list commands.
Eventually, the complete `do_pick` interface will be exposed to the
user in some form or another and those commands will become simple
aliases for the `do_pick` options now used to implement them.

Add the git-commit option `--reset-author` to the options pool of
`do_pick`. It rewrites the author date and name of the picked commit
to match the committer date and name.

If `--reset-author` is passed to `do_pick`, set the `rewrite` flag
and relay the option to the git-commit command line which creates the
final commit. If `--amend` is not passed as well, the fresh
authorship effect is achieved by the mere fact that we are creating a
new commit.
rebase -i: Teach do_pick the option --signoff

`do_pick` is the git-cherry-pick wrapper in git-rebase--interactive
that is currently used to implement most of the to-do list commands
and offers additional options that will eventually find their way
onto to-do lists.

To extend the repertoire of available options, add the git-commit and
git-cherry-pick option `--signoff` to the `do_pick` interface. It
appends a Signed-off-by: line using the committer identity to the log
message of the picked commit.
rebase -i: Enable options --signoff, --reset-author for pick, reword

pick and reword are atomic to-do list commands in the sense that they
open a new task which is closed after the respective command is
completed. squash and fixup are not atomic. They create a new task
which is not completed until the last squash or fixup is processed.

Lift the general unknown option blockade for the pick and reword
commands. If `do_cmd` comes across one of the options `--signoff` and
`--reset-author` while parsing a to-do entry and the scheduled
command is either `pick` or `reword`, relay the option to `do_pick`.

The `do_pick` options `--gpg-sign` and `--file` are not yet supported
because `do_cmd` cannot handle option arguments and options with
spaces at the moment. It is true that edit is one of the atomic
commands but it displays hash information when the rebase is stopped
and some options rewrite the picked commit which alters that
information. squash and fixup still do not accept user options as the
interplay of `--reset-author` and the author script are yet to be
determined.
rebase -i: Teach do_pick the option --gpg-sign

`do_pick` is the git-cherry-pick wrapper in git-rebase--interactive
that is used to replay the commits mentioned on to-do lists. The
git-rebase interface comes with several options that change how
commits are replayed (for instance, there is a option that asks
git-rebase to sign all replayed commits with the user's GPG key).

To make the replay of commits internally better configurable, add a
new option `--gpg-sign`, which is available from both the git-commit
and the git-cherry-pick interface, to `do_pick`. It appends a GPG
signature to the named commit after it is replayed. The private key
used for signing can either be passed as an argument to the option or
left to the user's gnupg configuration.
rebase -i: Handle rebase option --gpg-sign in do_pick

If `--gpg-sign` is passed to git-rebase, all replayed commits are
GPG-signed. In interactive mode, pretend `--gpg-sign` is passed to
every to-do list command by relaying `gpg_sign_opt` to `do_pick` in
`do_next`. `gpg_sign_opt` is either of the format `-S` or `-S<keyid>`
which are both supported by `do_pick`.