R1:016b060aeafd

Add a relation table for Revisions to local commit hashes

Summary:
This allows us to performantly query for diffs related to a given local hash.
Immediate applications are:

- Commit detection in Mercurial and Git-Immutable workflows.
- Some async unit test stuff @mgummelt was doing.

Test Plan:
Diffed locally under SVN/Git/hg, checked the table, got sensible output.

mysql> select * from differential_revisionhash;
+------------+------+------------------------------------------+
| revisionID |…