Diff any two files (BB-14652)

We have a conversion scenario where new files are being created in one directory based on files in another (but not outright replacing the old files, hence not just editing them).

People are keen to diff the original against the final, which effectively means an arbitrary diff of two files in a repo.

That is, they'd like to compare:

path1/foo.ext
path2/bar.ext

Currently the diff is within the same branch, but the more powerful the better.

While not a critical feature people are keen to use the same diff tool as much as possible and it would be nifty. It is possible to quickly make a snippet and view revisions, but it's a little clunky.