Filter commits to only those affecting the project

[nomeata]

Just another half-whacky idea…

The changelog is very useful. It would be more useful, though (especially when updating nixpkgs or the like) if it would only list those commits that actually affect “me”.

I wrote a tool called git-multisect to implement some of the logic (see [the example with nix flakes]](https://github.com/nomeata/git-multisect#a-realistic-example)). This idea could be adopted to work with niv (thanks to NIV_OVERRIDE) as well.

It would probably need the user of the action to specify one or more attributes to evaluate, so that only those commits are listed where any of these attributes now evaluate to different derivations (no need to build them).

If they don’t change at all, maybe the whole PR bump can be skipped.

(I’m currently not that actively working with projects using niv-auto-updater that I’ll PR that idea right away. Maybe in the future. Until then I just wanted to note it down here, I hope that’s ok.)