sequencer: allow editing the commit message on a case-by-case basis

sequencer: allow editing the commit message on a case-by-case basis

In the upcoming commits, we will implement more and more of rebase -i's
functionality inside the sequencer. One particular feature of the
commands to come is that some of them allow editing the commit message
while others don't, i.e. we cannot define in the replay_opts whether the
commit message should be edited or not.

Let's add a new parameter to the run_git_commit() function. Previously,
it was the duty of the caller to ensure that the opts->edit setting
indicates whether to let the user edit the commit message or not,
indicating that it is an "all or nothing" setting, i.e. that the
sequencer wants to let the user edit *all* commit message, or none at
all. In the upcoming rebase -i mode, it will depend on the particular
command that is currently executed, though.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>