t4150: am with applypatch-msg hook

t4150: am with applypatch-msg hook

Since d1c5f2a (Add git-am, applymbox replacement., 2005-10-07), git-am
will invoke the applypatch-msg hooks just after extracting the patch
message. If the applypatch-msg hook exits with a non-zero status, git-am
abort before even applying the patch to the index.

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>