repo: Bump mtime any time we write a ref

External daemons like rpm-ostree want push notification any time a
change is made by an external entity.  inotify provides notification,
but a problem is there's no easy way to monitor all of the refs.

In the past, there has been discussion of opt-in recursive timestamps:
https://lkml.org/lkml/2013/4/5/307

But in today's world, let's just bump the mtime on the repo itself, as
a central inotify point.

Closes: https://github.com/GNOME/ostree/pull/111

src/libostree/ostree-repo-refs.c

@@ -631,6 +631,9 @@ _ostree_repo_write_ref (OstreeRepo    *self,
         goto out;
     }
 
+  if (!_ostree_repo_update_mtime (self, error))
+    goto out;
+
   ret = TRUE;
  out:
   return ret;