Fix bomb when showing differences between two diff versions

Fix crash when trying to show the differerence between two versions of a patch
(e.g. with a URL like r/31/diff/2-3/).  RawFileDiffData was passing self.binary
to tool.get_parser(), which fails because the buffer object has no splitlines()
method.  Fixed by passing self.content instead.

Testing Done:
Ad-hoc testing - I now see a diff rather than a stack trace.

Reviewed at https://reviews.reviewboard.org/r/6297/