rebase -i: fix post-rewrite hook with failed exec command

rebase -i: fix post-rewrite hook with failed exec command

Usually, when 'git rebase' stops before completing the rebase, it is to
give the user an opportunity to edit a commit (e.g. with the 'edit'
command). In such cases, 'git rebase' leaves the sha1 of the commit being
rewritten in "$state_dir"/stopped-sha, and subsequent 'git rebase
--continue' will call the post-rewrite hook with this sha1 as <old-sha1>
argument to the post-rewrite hook.

The case of 'git rebase' stopping because of a failed 'exec' command is
different: it gives the opportunity to the user to examine or fix the
failure, but does not stop saying "here's a commit to edit, use
--continue when you're done". So, there's no reason to call the
post-rewrite hook for 'exec' commands. If the user did rewrite the
commit, it would be with 'git commit --amend' which already called the
post-rewrite hook.

Fix the behavior to leave no stopped-sha file in case of failed exec
command, and teach 'git rebase --continue' to skip record_in_rewritten if
no stopped-sha file is found.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>