Skip uninteresting commits for blame

[cruessler]

This PR is a PoC. It is a first attempt at implementing custom graph traversal for blame::file. This allows us to skip commits that we know don’t contain changes to the file being blamed. The PR is rough around the edges and contains a lot of TODOs and todo!()s.

A first round of tests showed a significant reduction in the number of commits traversed for gix blame STABILITY.md:

# before
Statistics {
    commits_traversed: 9261,
    commits_to_tree: 10710,
    trees_decoded: 10710,
    trees_diffed: 37,
    blobs_diffed: 36,
}

# after
Statistics {
    commits_traversed: 3657,
    commits_to_tree: 3749,
    trees_decoded: 3749,
    trees_diffed: 37,
    blobs_diffed: 36,
}

Also, profiling gix blame STABILITY.md on my machine revealed that the command took about 20 % less time to run.

[cruessler]

I think (and hope :-)) that this PR is now ready for the first round of reviews. There’s still two or three todo!() left, but I’ve addressed all of the TODOs as far as I can see and I’m really satisfied with the numbers so far as there seems to be a considerable speedup.

Also, the traversal is now much closer to what git does, and I hope that it most of the time is even identical unless there are differences in diffing results that can make traversals diverge. I haven’t found ways to validate this claim at a large scale, though, so I want to be cautious at this point.

gitoxide on  main [!?⇣] is 📦 v0.40.0 via 🦀 v1.84.0 took 5s
❯ hyperfine "$HOME/github/Byron/gitoxide/target/release/gix blame STABILITY.md" "$HOME/worktrees/gitoxide/branch-1/target/release/gix blame STABILITY.md"
Benchmark 1: /home/christoph/github/Byron/gitoxide/target/release/gix blame STABILITY.md
  Time (mean ± σ):     194.6 ms ±   2.3 ms    [User: 174.0 ms, System: 19.8 ms]
  Range (min … max):   190.8 ms … 198.5 ms    15 runs

Benchmark 2: /home/christoph/worktrees/gitoxide/branch-1/target/release/gix blame STABILITY.md
  Time (mean ± σ):      99.2 ms ±   1.5 ms    [User: 87.3 ms, System: 11.3 ms]
  Range (min … max):    97.1 ms … 104.1 ms    29 runs

Summary
  '/home/christoph/worktrees/gitoxide/branch-1/target/release/gix blame STABILITY.md' ran
    1.96 ± 0.04 times faster than '/home/christoph/github/Byron/gitoxide/target/release/gix blame STABILITY.md'

[Byron]

That's a massive speedup, fantastic work! And sorry for the late (first) review, I am now totally behind on emails each, catching up only during the weekend.

When looking at the commits I saw that they are a bit all over the place, often touching multiple crates at the same time. This is OK when not using <prefix>: <subject> notation, but it's not otherwise.

Can we practice this by adjusting the <prefix>: <subject> commits so that they only touch a single crate? This can be followed by an adapt to changes in <crate> commit to make fixes that make it compile (yes, unfortunately not every commit would pass CI, but it's for getting proper changelogs).

In that moment, please feel free to squash other commits that might not be so valuable all by themselves.

I am also more than happy to do this as part of my review, just let me know.

[cruessler]

Sure, no problem, thanks a lot for the detailed explanation! (I wasn’t aware that this means accepting intermediate commits that don’t pass CI.)

I will rearrange the commits, but I will only get to it on Monday or Tuesday, so feel free to take over if you get to it earlier!