Merge branch 'mh/ref-lock-entry'

The code to acquire a lock on a reference (e.g. while accepting a push from a client) used to immediately fail when the reference is already locked---now it waits for a very short while and retries, which can make it succeed if the lock holder was holding it during a read-only operation. * mh/ref-lock-entry: refs: retry acquiring reference locks for 100ms