[1.x] Adds --diff option

[ProjektGopher]

This PR is how I've always wished the --dirty flag worked.

pint --dirty will lint staged and unstaged files (and possibly untracked?). But once a file is committed, it will not be touched again by pint --dirty, whether or not it passes. The only time it will be linted is when we run pint on the whole project again.

This also means that running pint --dirty in CI is pointless, since all the files have already been committed, meaning we'll lint nothing. But running Pint without any flags can take upto five minutes in a large project, and that feedback cycle sucks.

pint --diff=main will collect all committed, staged, unstaged, and untracked files that differ from the provided branch. Meaning that if we've got a PR that only touches 5 files, we can now run pint --diff=main in CI, and we're only going to look at those 5 files, saving a ton of time.

We can also use this effectively locally. If your regular workflow is to create a branch from within a new issue, then check that branch out locally, your main branch might not be up to date with origin. Running pint --diff=main might not give you the results you'd expect then. However using pint --diff=origin/main can help speed up your feedback cycle dramatically.

Note

I added a new method to the PathsRepository contract, which could be considered a breaking change.
Here's the commit to revert if you need to:

git revert e295403a66c145e9b5530d3e5f9614875fc90162

[taylorotwell]

@ProjektGopher Thanks! Could you send a documentation PR for this? 👍