R1:021c612cb228

When we fail to acquire a repository lock, try to provide a hint about why

Summary:
Ref T13202. See PHI889. If the lock log is enabled, we can try to offer more details about lock holders.

When we fail to acquire a lock:

- check for recent acquisitions and suggest that this is a bottleneck issue;
- if there are no recent acquisitions, check for the last acquisition and print details about it (what process, how long ago, whether or not we believe it was released).

Test Plan:
- Enabled the lock log.