buffer: ensure objects are added in topological order
This new integration test checks that, when flushing the buffer storage, the addition functions of the underlying storage backend are called in topological order (content, directory, revision, release then snapshot).
This reduces the probability of "data consistency" regressions caused by the use of the buffering storage proxy alone.
Depends on !641 (closed)
Migrated from D5017 (view on Phabricator)