Don't fatal on missing commit in DiffusionBrowseFileController

Summary:
Still not 100% sure what the repro case here is, but we do something similar on line 438 above. One case could be an SVN repo with a subpath specified, I think.

In any case, don't fatal if we're missing the commit.

Test Plan: Loaded some browse views, although I don't have a repro.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2959