am: support --allow-empty to record specific empty patches

am: support --allow-empty to record specific empty patches

This option helps to record specific empty patches in the middle
of an am session, which does create empty commits only when:

    1. the index has not changed
    2. lacking a branch

When the index has changed, "--allow-empty" will create a non-empty
commit like passing "--continue" or "--resolved".

Signed-off-by: 徐沛文 (Aleen) <aleen42@vip.qq.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>