The DiffFile model (and the files context key when rendering

diffviewer templates) still used variable names from the very first days

of Review Board, when we used a fair bit of Perforce terminology.
This change fixes that to use orig_filename, orig_revision,

modified_filename, and modified_revision. This is a breaking change,

but I think that's okay for two reasons:

The DiffFile model is pretty much an implementation detail of our

   diffviewer UI. It seems extremely unlikely that anybody would be

   digging into this.
The documentation inside DiffFile was quite incorrect for many of

   the attributes. This was fixed in a prior change, but reinforces my

   belief that nobody was using it.

Ran python tests.

Ran js-tests.