When git-am is first run on an unborn branch, no ORIG_HEAD is created.
As such, any applied commits will remain even after a git am --abort.
To be consistent with the behavior of git am --abort when it is not run
from an unborn branch, we empty the index, and then destroy the branch
pointed to by HEAD if there is no ORIG_HEAD.
Signed-off-by: Paul Tan <pyokagan@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>