Make sure diff always uses the default diff driver when create_patch=True

[can-taslicukur]

Closes #1828

This change forces diff to always use the default diff driver when create_patch=True. So any user using external diff can reproduce the same result as users that don't.

[Byron]

Thanks a lot! Much better than overriding the configuration like I suggested!

Would you mind digging into the test-suite and try to adjust one of them so that it will, indeed, set the diff.external value to some bogus program that makes the test fail unless the --no-ext-diff line is present?

It shouldn't be too hard and will assure this won't ever regress. This woudl also mean the comment can then be removed (as removing the statement will cause test failure).

Thanks for your consideration.

[can-taslicukur]

Thank you so much for the review @Byron!

I've included a new test, inspired by @EliahKagan 's steps to reproduce the problem in the issue and my own situation. These tests only pass if we set the --no-ext-diff flag when we create patch. PTAL!

[Byron]

Thanks so much for your continued involvement and the test, it's great and much more thorough than I envisioned it!