oxidizing gitui (migration to `gitoxide`)

This is a tracking issue for oxidizing `gitui`. I’ve only collected the areas that already have at least one PR. I want to add more context and details over time. I hope I didn’t miss any existing tracking issue on the same topic. :smile:

### High-level

- [ ] status
- [x] tree view (#2673)
- [ ] diff view (#2685)
- [x] tags (#2664)
- [ ] branches (#2706)
- [ ] blame (#2591)
- [x] log view without filter (#2275)
- [ ] log view with filter

### Low-level

- [x] `get_commit_info` (#2654)

### Clean-up/refactor

Some of the following refactorings might not be possible yet, but depend on other work to be finished before we can start working on them.

- [x] extract function `gix_repo()`, similar to `repo()`, that wraps current `gix::ThreadSafeRepository::discover_with_environment_overrides(repo_path.gitpath()).map(Into::into)?`
- [ ] either remove `untracked_files_config_repo` in case it is not needed anymore or change it to match git behaviour
- [x] extract `gitoxide`-related errors into a separate error variant `GixError` to keep them in a single place (#2687)