Allow edit of empty message with commit --amend

Allow edit of empty message with commit --amend

"git commit --amend" used on a commit with an empty message fails
unless -m is given, whether or not --allow-empty-message is
specified.

Allow it to proceed to the editor with an empty commit message.
Unless --allow-empty-message is in force, it will still abort later
if an empty message is saved from the editor (this check was
already necessary to prevent a non-empty commit message being edited
to an empty one).

Add a test for --amend --edit of an empty commit message which fails
without this fix, as it's a rare case that won't get frequently
tested otherwise.

Signed-off-by: Chris Webb <chris@arachsys.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>