sequencer: prepare for rebase -i's commit functionality

sequencer: prepare for rebase -i's commit functionality

In interactive rebases, we commit a little bit differently than the
sequencer did so far: we heed the "author-script", the "message" and the
"amend" files in the .git/rebase-merge/ subdirectory.

Likewise, we may want to edit the commit message *even* when providing a
file containing the suggested commit message. Therefore we change the
code to not even provide a default message when we do not want any, and
to call the editor explicitly.

Also, in "interactive rebase" mode we want to skip reading the options
in the state directory of the cherry-pick/revert commands.

Finally, as interactive rebase's GPG settings are configured differently
from how cherry-pick (and therefore sequencer) handles them, we will
leave support for that to the next commit.

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