The get_files_affected_by_patch method on the test API will return a
TestData object that determines the output of the "git diff" call. a
Test was added specifically for the get_files_affected_by_patch API
method to ensure that the test API operates as expected.

Change-Id: Ia1846700461757a72ed518894a5a38a8ca54fcc9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3422006
Auto-Submit: Garrett Beaty <gbeaty@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Garrett Beaty <gbeaty@google.com>