hook API: add a run_hooks() wrapper

Add a run_hooks() wrapper, we'll use it in subsequent commits for the
simple cases of wanting to run a single hook under a given name,
without providing options such as "env" or "args".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Emily Shaffer <emilyshaffer@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

hook.c

@@ -142,3 +142,10 @@ cleanup:
 	run_hooks_opt_clear(options);
 	return ret;
 }
+
+int run_hooks(const char *hook_name)
+{
+	struct run_hooks_opt opt = RUN_HOOKS_OPT_INIT;
+
+	return run_hooks_opt(hook_name, &opt);
+}