Allow specifying against commit in DiffusionRawDiffQuery

Summary: I will need this for tracking line number in Blame previous revision.

Test Plan:

$ hg diff --rev 0:1
$ svn diff -r 64382:64383 README

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3083