Compare breakdowns of modules between branches

[romaricpascal]

Part of #3279

What

Compare breakdown of modules in PR between the tip of the branch it'll be merged into.

Potentially reduce the comment on the PR to list only what's changed (the full breakdown would still be available in the review app).

Why

Comparing manually is possible, but requires us to remember to do it at all times. Doing it automatically will help ensure it happens and let us focus on what changed rather than spend effort figuring what's the same and what's different.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• Module content is compared on PR ¹
• Possibly PR comment is updated to only show what changed
• Something is in place to handle the base branch begin updated ²

Footnotes

1. This will likely call for keeping track of the module contents on the base branch rather than re-computing each time (either in Github action cache, as Git note or something else) ↩

2. Github will re-run workflows when we push to the PR branch. If something changes on the base branch, though, we'll need to find a way to re-compute things (either automatically or have some info like the Git SHA that lets us infer that the stats are correct) ↩