Perform automatic diffs for pull requests

[zanieb]

As described in #460 (comment)

It'd be great to post a summary of changes to distributions on pull requests.

[indygreg]

Most bang for your buck is diffing the PYTHON.json file, as it encapsulates the most important bits, including the Python autoconf variables. Next most useful thing is the set of files whose content changed. Builds are highly deterministic. (And it should be possible to close remaining determinism gaps pretty easily to reduce noise.) So the changed files set should be pretty small for most PRs.