lib/commit: Add repository locking during transactions

Take a shared repo lock during a transaction to ensure that another
process doesn't delete objects.

Closes: #1343
Approved by: cgwalters

src/libostree/ostree-repo-private.h

@@ -122,6 +122,7 @@ struct OstreeRepo {
 
   GMutex txn_lock;
   OstreeRepoTxn txn;
+  gboolean txn_locked;
 
   GMutex cache_lock;
   guint dirmeta_cache_refcount;