Now check for existence of the original file in the repository only if a parent diff is NOT supplied, not when it is. Previously it was checking when a parent diff was supplied (and not when it wasn't), which is presumably not the correct behaviour. If a parent diff is supplied it is generally because the file doesn't exist in the master repository, so ReviewBoard shouldn't try to find it there.

This was preventing parent diffs from working with Mercurial.

Verified that parent diff now works with Mercurial diffs.
Verified that uploading a diff now fails if trying to upload a Mercurial diff without a parent, when the source revision is not in the master repository.