R1:92f974177919

Add a safety check to commit discovery daemon

Summary:
Although I couldn't repro the issue in T692, I did manage to point the "Diviner"
repository at the "Phabricator" working copy and screw some stuff up on
secure.phabricator.com.

Before discovering commits in a repository, ensure the 'origin' remote points at
the configured URI. This prevents issues where the working copy gets configured
to point at an existing (but incorrect) checkout.

Test Plan:
- Ran gitcommitdiscovery daemon normally under "phd…