builtin-am: handle stray state directory

builtin-am: handle stray state directory

Should git-am terminate unexpectedly between the point where the state
directory is created, but the "next" and "last" files are not written
yet, a stray state directory will be left behind.

As such, since b141f3c (am: handle stray $dotest directory, 2013-06-15),
git-am.sh explicitly recognizes such a stray directory, and allows the
user to remove it with am --abort.

Re-implement this feature in builtin/am.c.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>