This change removes all RemovedInReviewBoard60 things from our codebase.

The only complicated part of this is that the mercurial diff parsers

were still using the old get_orig_commit_id method, so those have been

updated to set the data correctly on the ParsedDiffChange.
This also makes one small fix to the filediff API resource to handle a

deprecation removal in Djblets.

Ran unit tests.