am, rebase--merge: do not overlook --skip'ed commits with post-rewrite

am, rebase--merge: do not overlook --skip'ed commits with post-rewrite

The post-rewrite hook is supposed to be invoked for each rewritten
commit.  The fact that a commit was selected and processed by the rebase
operation (even though when we hit an error a user said it had no more
useful changes), suggests we should write an entry for it.  In
particular, let's treat it as an empty commit trivially squashed into
its parent.

This brings the rebase--am and rebase--merge backends in sync with the
behavior of the interactive rebase backend.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>