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-private.h

@@ -255,5 +255,9 @@ _ostree_repo_read_bare_fd (OstreeRepo           *self,
                            int                  *out_fd,
                            GCancellable        *cancellable,
                            GError             **error);
+
+gboolean
+_ostree_repo_update_mtime (OstreeRepo        *self,
+                           GError           **error);
                            
 G_END_DECLS