We have these SHA hashes for file diffs that are useful to checksum the
file diffs. They exist in the file diff's extra data and as attributes
on the object, but aren't officially listed in our API. It'd be useful
to expose them in the API so that callers can make use of them without
having to reach into the extra data and know the internal key naming of
the SHAs.

Ran unit tests.
Used in an upcoming RBTools change where I access the patched_sha256

  attribute via the API.