The FileDiff sorting used when generating the list of files in the diff

viewer was all done inside of get_diff_files(), and the way it was

designed required that we store the base name and base path of each file

in the cache, despite that data never being used after sorting that data

pre-cache.
That logic has now been moved out into a get_sorted_filediffs()

function. This handles the computation of the base name and base path,

which are only computed once per entry (as per the documentation on

sorted()'s key parameter), and are not retained after the sort

operation.
We'll be able to use this function in other places that may need to

represent lists of files.

Tested before/after with some diffs and didn't see any change in the

file listings.
Unit tests pass.