rebase: invoke post-rewrite hook

rebase: invoke post-rewrite hook

We have to deal with two separate code paths: a normal rebase, which
actually goes through git-am; and rebase {-m|-s}.

The only small issue with both is that they need to remember the
original sha1 across a possible conflict resolution.  rebase -m
already puts this information in $dotest/current, and we just
introduce a similar file for git-am.

Note that in git-am, the hook really only runs when coming from
git-rebase: the code path that sets the $dotest/original-commit file
is guarded by a test for $dotest/rebasing.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>