feat: log: search changes

[PRESFIL]

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

[altsem]

I reckon this becomes quite expensive, diffing each commit to the previous. It does take a few seconds to complete a search for me in the Gitu repo with a release-build.

I've been replacing other parts of (lib)git2 uses with simply command forks. git log --oneline --grep 'test' might do the job. Perhaps there's a lower-level, more stable command than git log --grep that is more suitable for scripting (aka plumbing-command). Would require some rework.