Github API v3 - Fetch diff of specific commit

Github API v3 - Fetch diff of specific commit

I'm trying to figure out how to fetch the diff from a specific commit on
Github. (Basically, just a series of individual file diffs, associated
with a particular commit).
Some of the links on this question helped me figure out how to pull the
contents of a given file in a repo, but I can't figure out how to pull the
file for a particular commit.
OR, more to the point, how to pull the whole diff (pretty much as
displayed when looking at a particular commit on Github).
Is there an easy way I'm missing to do this with the API? Or a Github gem
for Rails that can do something like this? Took a look at the Git gem, but
it doesn't seem to be what I'm looking for.