builtin-am: invoke 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.

Re-implement this in builtin/am.c.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/am.c

@@ -1378,6 +1378,8 @@ static void do_commit(const struct am_state *state)
 		fclose(fp);
 	}
 
+	run_hook_le(NULL, "post-applypatch", NULL);
+
 	strbuf_release(&sb);
 }