t4150: am with pre-applypatch hook

t4150: am with pre-applypatch hook

Since d1c5f2a (Add git-am, applymbox replacement., 2005-10-07),
git-am.sg will invoke the pre-applypatch hook after applying the patch
to the index, but before a commit is made. Should the hook exit with a
non-zero status, git am will exit.

Add tests for this hook.

Helped-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>