Fix issue with _process_files being too strict in its type-checking.
Fix issue with _process_files being too strict in its type-checking.

A recent change went in to ensure that various things were the correct
string type, but the checking for the result of
`SCMTool.parse_diff_revision` was too strict, allowing only `bytes` but
not `Revision` objects. This would cause diff validation errors when
attempting to post newly-added files.

Testing Done:
Was able to post a change including a newly-added file.

Reviewed at https://reviews.reviewboard.org/r/10549/

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