Fix issue with _process_files being too strict in its type-checking.
Review Request #10549 — Created May 9, 2019 and submitted
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_revisionwas too strict, allowing only
Revisionobjects. This would cause diff validation errors when
attempting to post newly-added files.
Was able to post a change including a newly-added file.