The git diff command is pretty awesome - diffing any commit on any repository.
However, it kinda sucks from a UI/UX perspective - and esp. if you want to share the results (for example, with someone who has no git with them) - it's just not very cloudy.
Ages ago it appears github introduced a major enhancement to their comparison functionality that I only just spotted (so apologies if you seen this).
Full info is here on github's page for it - https://github.com/blog/683-cross-repository-compare-view - but the full beauty (and immediately obvious explanation) is here:
With the resulting page you can share the comparison by just copying the URL, like this one below!