rebase -i: Teach do_pick the option --signoff

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

Add `--signoff` to the options pool of `do_pick`. It appends a
Signed-off-by: line using the committer identity to the log message of
the picked commit.

If `--signoff` is passed to `do_pick`, set the `rewrite` flag and relay
the option to the git-commit command line which creates the commit after
the picked changes are applied to the working directory and index.