PR descriptions link to update's source diff

[KiaraGrouwstra]

Going by e.g. NixOS/nixpkgs#486281, the PR description template seems to currently not include links to source diffs, instead settling for release notes and change logs.

To facilitate verification of source changes, a potential improvement would seem including such links, for upstream forges supporting this.

c.f. the (similar-sounding) #45, apparently closed in favor of the (seemingly vanished) #59.

[rhendric]

For any future (human!) contributors who wish to address this issue, note that the reason that NixOS/nixpkgs#486281 has no diff link is nothing to do with the upstream forge — this upstream is hosted on GitHub. Rather, the issue is that the source derivation fetches submodules, and that changes the available attributes on src. Nix.getSrcUrl would need to be enhanced to cover this case.

(From a very unsystematic sampling of recent PRs, it seems to me that the most common cause of this diff link going missing is neither submodules nor foreign forges, but the recent-ish adoption of refs/tags/TAGNAME.tar.gz-style URLs. It would be a simple change to GH.parseUrlMaybe to support these, and that's the direction in which I'd point a potential contributor eager to get the most bang for their buck.)