rebase -i: invoke post-rewrite hook

rebase -i: invoke post-rewrite hook

Aside from the same issue that rebase also has (remembering the
original commit across a conflict resolution), rebase -i brings an
extra twist: We need to defer writing the rewritten list in the case
of {squash,fixup} because their rewritten result should be the last
commit in the squashed group.

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