R1:c402d7d3076d

Implement smart waits for rarely updated repositories

Summary:
Ref T4605. When figuring out how long to wait to update a repository, factor in when it was last pushed. For rarely updated repositories, wait longer between updates.

(A slightly funky thing about this is that empty repos update every 15 seconds, but that seems OK for the moment.)

Test Plan:
Ran `bin/phd debug pulllocal` and saw sensible calculations and output:

```
...
<VERB> PhabricatorRepositoryPullLocalDaemon Last commit to repository…