Fix hg parent diffs (again?).

If a standard hg diff is posted, along with a parent diff, and the diff
touches files that the parent diff didn't, application will fail. This
change now properly fills the source_revision field on these filediffs
so that we don't attempt to fetch the file for the revision the parent
diff created.

The revision handling for the hg case is messy and could really use some
more love to make things more sensical and robust. I'm going to hold off
on any big changes for now since I think it's important to just get this
use case working.

Reviewed at https://reviews.reviewboard.org/r/6253/