This change adds diff unit tests for all the TFS wrappers, helping

ensure we have as close to comprehensive diff coverage as possible. This

will be important for upcoming work on diff tools, and just ensures we

don't have any type-related issues.
This did uncover that we were fetching content for binary files when we

didn't need to. These were being fetched and then ignored. These code

paths have been updated to avoid the fetch for binary files.
The rb-tfs wrapper performs its own diffing, so we can't test or even

work with any of that. Ideally, that would be updated to provide

information we could use to generate a diff internally, but that's far

outside the scope of the work for RBTools 4.

Unit tests pass on Python 3.7-3.11.