Move ref writing to be transaction-based

Rather than having separate write_ref calls, make clients start a
transaction, add some refs, and then commit it. While this doesn't
make it 100% atomic, it makes it easier for us to use an atomic
model, and it means we don't do as much I/O updating the summary
file and such.

https://bugzilla.gnome.org/show_bug.cgi?id=707644

src/libostree/ostree-repo.c

@@ -100,6 +100,7 @@ ostree_repo_finalize (GObject *object)
     g_hash_table_destroy (self->updated_uncompressed_dirs);
   if (self->config)
     g_key_file_free (self->config);
+  g_clear_pointer (&self->txn_refs, g_hash_table_destroy);
   g_clear_pointer (&self->cached_meta_indexes, (GDestroyNotify) g_ptr_array_unref);
   g_clear_pointer (&self->cached_content_indexes, (GDestroyNotify) g_ptr_array_unref);
   g_mutex_clear (&self->cache_lock);