Diffstat for PRs

[dgw]

Maybe useful to include diffstat (+/- line count, # of files changed) in PR link output?

Webhooks include this info when a PR is opened ("additions", "deletions", and "files_changed" keys are directly inside the "pull_request"). Commit count is obviously also available, but probably less useful than the space it would consume in the output IRC line (debatable, of course).

Showing it for linked PRs would be a bit more annoying, as it requires specifically calling the pulls API endpoint instead of the issues one to get the relevant keys. Would have to complicate the logic/matching for issue_info() somewhat.