wip: blame

[altsem]

TODO

• Will want to show auther/date right-aligned
• enter on a commit in the blame-view should open that commit at that line
• Blaming something in a commit should blame from that version
• Blaming a hunk or line within a hunk should also be possible

Checklist

• The modified code has some test-coverage (where applicable).
• make test is passing (this is what CI runs).
• New unreleased features/fixes/styling and performance improvements are documented via git: feat: / fix: / style: / perf:. Or e.g. perf(highlighting):.
  See https://github.com/altsem/gitu/blob/master/docs/dev-tooling.md