Fix diff output for renames and copies

If you use rename detection, the renamed and copied files would
not show any text diffs because the function that decides if
data should be loaded didn't know which sides of the diff to
load for those cases.

This adds a test that looks at the patch generated for diff
entries that are COPIED or RENAMED.
Merge pull request #1572 from arrbee/diff-patch-with-rename

Fix diff output for renames and copies