t4150: am with post-applypatch hook

t4150: am with post-applypatch hook

Since d1c5f2a (Add git-am, applymbox replacement., 2005-10-07),
git-am.sh will invoke the post-applypatch hook after the patch is
applied and a commit is made. The exit code of the hook is ignored.

Add tests for this hook.

Helped-by: Junio C Hamano <redacted>
Reviewed-by: Stefan Beller <redacted>
Reviewed-by: Johannes Schindelin <redacted>
Signed-off-by: Paul Tan <redacted>
Signed-off-by: Junio C Hamano <redacted>