R1:e478c0074ca3

Add support for querying by commit hashes to DifferentialRevisionQuery class and
corresponding ConduitAPI

Summary: reasonable title... also made this new functionality used by the
repository worker for parsing diffs

Test Plan:
- looked at the conduit console and queried for various types of hashes,
including hashes with no match. got correct results.
- identified a reasonable diff from a local git repo. set the revision status
to 2 (ACCEPTED) in the database. augmented the worker parser code to…