commit: honour --no-edit

After making fixes to the contents to be committed, it is not unusual to
update the current commit without rewording the message. Idioms to tell
"commit --amend" that we do not need an editor have been:

    $ EDITOR=: git commit --amend
    $ git commit --amend -C HEAD

but that was only because a more natural "--no-edit" option in

    $ git commit --amend --no-edit

was not honoured.

Signed-off-by: Junio C Hamano <gitster@pobox.com>