repo: Make abort_transaction silently succeed if we're not in a transaction

This helps callers out a lot, and means we can always call abort_transaction
at the end of a function.

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

src/libostree/ostree-repo-commit.c

@@ -662,6 +662,9 @@ ostree_repo_abort_transaction (OstreeRepo     *self,
 {
   gboolean ret = FALSE;
 
+  if (!self->in_transaction)
+    return TRUE;
+
   if (!cleanup_tmpdir (self, cancellable, error))
     goto out;