Add support for relative files and rebasing

[palas]

Changelog

- description: |
    Added support for relative files and rebasing to the prettify tool
# uncomment types applicable to the change:
  type:
  - maintenance    # not directly related to the code

Context

When rebasing there was no option of prettify that would work (because changes while rebasing are neither staged nor unstaged, they are in the previous commit). So this PR adds an option -p that diffs against HEAD~1.

Also the tool would not work unless run from the top level in the repo, which is kind of inconvenient. So this change make prettify work from any subfolder.

Twin PR #589 in cardano-api

How to trust this PR

Probably testing is the best way to ensure it works. But they are few changes, so a quick look at the diff would also be nice.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff