builtin-am: invoke pre-applypatch hook

builtin-am: invoke 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.

Re-implement this in builtin/am.c.

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