commit --amend: invoke post-rewrite hook

The rough structure of run_rewrite_hook() comes from
run_receive_hook() in receive-pack.

We introduce a --no-post-rewrite option and use it to avoid the hook
when called from git-rebase -i 'edit'.  The next patch will add full
support in git-rebase, and we only want to invoke the hook once.

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

git-rebase--interactive.sh

@@ -445,7 +445,7 @@ do_next () {
 		mark_action_done
 		pick_one $sha1 ||
 			die_with_patch $sha1 "Could not apply $sha1... $rest"
-		git commit --amend
+		git commit --amend --no-post-rewrite
 		;;
 	edit|e)
 		comment_for_reflog edit