Add ratcheting (gradual adoptance)

[slarse]

We need to make it possible to only consider introduced violations, i.e. violations that are contained in edited source code. This is commonly referred to as "ratchet". I've used it with success with Spotless, and think we can draw inspiration from there.

[slarse]

I've found a few more things I need to implement ratchet:

• git.Repo.diff() must take an optional argument commitish which specifies what commit-ish to compare against
  • This is to enable custom ratchet-from targets
• It must be possible to get the worktree root of a repo, e.g. git.Repo.getWorktreeRoot()
  • This is necessary in order to match filepaths reported by git diff to those reported by Sorald
  • Can be implemented as git rev-parse --show-toplevel
• A function that checks closed ranges for overlap.
  • For example, given a range [10, 17] and another range [15, 20] there is overlap, but for [10, 17] and [8, 9], there is no overlap.