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.

Tested by posting a change with a parent diff that didn't touch files in

the main diff. Applying the diff failed before this patch, and succeeds

after.