builtin-am: invoke applypatch-msg hook

builtin-am: invoke applypatch-msg hook

Since d1c5f2a (Add git-am, applymbox replacement., 2005-10-07),
git-am.sh 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.sh abort before even applying the patch to the index.

Re-implement this in builtin/am.c.

Signed-off-by: Paul Tan <redacted>
Signed-off-by: Junio C Hamano <redacted>