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

builtin/am.c

@@ -1334,6 +1334,9 @@ static void do_commit(const struct am_state *state)
 	const char *reflog_msg, *author;
 	struct strbuf sb = STRBUF_INIT;
 
+	if (run_hook_le(NULL, "pre-applypatch", NULL))
+		exit(1);
+
 	if (write_cache_as_tree(tree, 0, NULL))
 		die(_("git write-tree failed to write a tree"));