sequencer: handle post-rewrite for merge commands

sequencer: handle post-rewrite for merge commands

In the previous patches, we implemented the basic functionality of the
`git rebase -i --rebase-merges` command, in particular the `merge`
command to create merge commits in the sequencer.

The interactive rebase is a lot more these days, though, than a simple
cherry-pick in a loop. For example, it calls the post-rewrite hook (if
any) after rebasing with a mapping of the old->new commits.

This patch implements the post-rewrite handling for the `merge` command
we just introduced. The other commands that were added recently (`label`
and `reset`) do not create new commits, therefore post-rewrite hooks do
not need to handle them.

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