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 var_dump
and die after finding revision id. ran scripts/repository/reparse.php
--message rX and verified my var_dumps. removed var_dumps and die and ran
reparse.php again with same paramters. verified revision looked good in
diffusion and there were no errors.

• repeated the above reparse.php jonx for a mercurial repo. note svn isn't in

this hash game so that test was particularly exciting no-op'dness i did not
bother with

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, btrahan, epriestley

Differential Revision: https://secure.phabricator.com/D1315