[RFC PATCH 2/7] rebase -i: Teach do_pick the option --edit

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. Teach `do_pick` to handle the option `--edit` and reimplement `reword` in terms of `do_pick --edit`. Refer to `pick_one` for the way options are parsed.

`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. Adjust the fragment in two regards. Firstly, discard the additional message which is printed if an error occurs because

    Aborting commit due to empty commit message. (Duplicate Signed-off-by lines.)
    Could not amend commit after successfully picking 1234567... Some change

is more readable than and contains (almost) the same information as

    Aborting commit due to empty commit message. (Duplicate Signed-off-by lines.)
    Could not amend commit after successfully picking 1234567... Some change
    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.

(It is true that a hook might not output any diagnosis but the same problem arises when using git-commit directly. git-rebase at least prints a generic message saying that it failed to commit.) Secondly, sneak in additional git-commit arguments:

 - `--allow-empty` is missing: `rebase--interactive` suddenly fails if
   an empty commit is picked using `reword` instead of `pick`. The
   whether a commit is empty or not is not changed by an altered log
   message, so do as `pick` does. Add test.

 - `-n`: Hide the fact that we are committing several times by not
   executing the pre-commit hook. Caveat: The altered log message is not
   verified because `-n` also skips the commit-msg hook. Either the log
   message verification must be included in the post-rewrite hook or
   git-commit must support more fine-grained control over which hooks
   are executed.

 - `-q`: Hide the fact that we are committing several times by not
   printing the commit summary.

Signed-off-by: Fabian Ruch <bafain@gmail.com>
---
 git-rebase--interactive.sh    | 52 ++++++++++++++++++++++++++++++++++++-------
 t/t3404-rebase-interactive.sh |  8 +++++++
 2 files changed, 52 insertions(+), 8 deletions(-)