Reallow git-rebase --interactive --continue if commit is unnecessary

Reallow git-rebase --interactive --continue if commit is unnecessary

During git-rebase --interactive's --continue implementation we used
to silently restart the rebase if the user had made the commit
for us.  This is common if the user stops to edit a commit and
does so by amending it.  My recent change to watch git-commit's
exit status broke this behavior.

Thanks to Bernt Hansen for catching it in 1.5.4-rc1.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>