Don't barf on bad commit identifiers

Summary:
If someone provides an invalid svn rev number (like providing a git commit
hash instead) for a diffusion commit, we should ignore it like we ignore
other bad input to DiffusionCommitQuery, instead of barfing.

Test Plan:
put an invalid blame rev with rEsomehash (where E is an svn repo), and
differential loads.

Reviewers: epriestley, wez

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5389