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

builtin/am.c

@@ -455,6 +455,27 @@ static void am_destroy(const struct am_state *state)
 	strbuf_release(&sb);
 }
 
+/**
+ * Runs applypatch-msg hook. Returns its exit code.
+ */
+static int run_applypatch_msg_hook(struct am_state *state)
+{
+	int ret;
+
+	assert(state->msg);
+	ret = run_hook_le(NULL, "applypatch-msg", am_path(state, "final-commit"), NULL);
+
+	if (!ret) {
+		free(state->msg);
+		state->msg = NULL;
+		if (read_commit_msg(state) < 0)
+			die(_("'%s' was deleted by the applypatch-msg hook"),
+				am_path(state, "final-commit"));
+	}
+
+	return ret;
+}
+
 /**
  * Runs post-rewrite hook. Returns it exit code.
  */
@@ -1420,6 +1441,9 @@ static void am_run(struct am_state *state, int resume)
 			write_commit_msg(state);
 		}
 
+		if (run_applypatch_msg_hook(state))
+			exit(1);
+
 		say(state, stdout, _("Applying: %.*s"), linelen(state->msg), state->msg);
 
 		apply_status = run_apply(state, NULL);