Improve Nixpkgs diff report

[infinisil]

The current Nixpkgs diff report is rather sparse, we should improve it.

[infinisil]

Here's a mock-up of how it could look:

---

Nixpkgs diff report

This shows how the changes in this PR affect Nixpkgs. This comment is continuously updated as the PR is updated and results become available. This Git branch is used for caching and can also be used to check the progress, all other links are immutable.

Using NixOS/nixpkgs@4b7a689 ¹ formatted using b84dba1 ² as the base.

Individual commits ³

1. 5c5beaa (Automated Nixpkgs diffs for PRs): Nixpkgs diff +50 -40 (+1%)
2. 5c5beaa (Some non-working commit): ❌ Build failure
3. 5c5beaa (Some commit subject): Nixpkgs diff +40 -50 (-1%)
4. 5c5beaa (latest commit): 🔄 In progress

All commits

• Nixpkgs diff (empty)

Footnotes

1. This is most most recent Nixpkgs commit before the first commit from this PR ↩

2. This is the first parent of the first commit from this PR ↩

3. Nixpkgs is reset to the base before running each commits formatter, such that intermediate commits don't affect the end result ↩

[infinisil]

Here's something that the report should also make clearer: #201 (comment)