gix-blame: Replace BTreeMap by SmallVec

[cruessler]

This PR increases gix-blame’s performance by replacing suspects: BTreeMap with suspects: SmallVec.

A SmallVec provides a better trade-off with respect to performance than a BTreeMap because suspects in most cases only contains 1 item. The cost of calling BTreeMap::insert and BTreeMap::remove becomes noticeable in profiling the longer a blame’s history gets and the more hunks a file is split into.

For smaller blames, the benefits are negligible, but for larger repos and larger files they are so significant that I’m worried I might have made a mistake trying to measure them. 😄

The following benchmarks compare this PR against main.

❯ env GIT_DIR="$HOME/github/Byron/gitoxide/.git" hyperfine --warmup 1 --export-markdown results.md '$HOME/github/Byron/gitoxide/target/profiling/gix blame Cargo.lock' '$HOME/worktrees/gitoxide/branch-6/target/profiling/gix blame Cargo.lock'
Benchmark 1: $HOME/github/Byron/gitoxide/target/profiling/gix blame Cargo.lock
  Time (mean ± σ):      1.482 s ±  0.009 s    [User: 1.284 s, System: 0.192 s]
  Range (min … max):    1.468 s …  1.499 s    10 runs

Benchmark 2: $HOME/worktrees/gitoxide/branch-6/target/profiling/gix blame Cargo.lock
  Time (mean ± σ):      1.269 s ±  0.008 s    [User: 1.078 s, System: 0.185 s]
  Range (min … max):    1.261 s …  1.289 s    10 runs

Summary
  '$HOME/worktrees/gitoxide/branch-6/target/profiling/gix blame Cargo.lock' ran
    1.17 ± 0.01 times faster than '$HOME/github/Byron/gitoxide/target/profiling/gix blame Cargo.lock'

❯ env GIT_DIR="$HOME/github/torvalds/linux/.git" hyperfine --warmup 1 --export-markdown results.md '$HOME/github/Byron/gitoxide/target/profiling/gix blame CREDITS' '$HOME/worktrees/gitoxide/branch-6/target/profiling/gix blame CREDITS'
Benchmark 1: $HOME/github/Byron/gitoxide/target/profiling/gix blame CREDITS
  Time (mean ± σ):      2.207 s ±  0.026 s    [User: 2.026 s, System: 0.174 s]
  Range (min … max):    2.179 s …  2.261 s    10 runs

Benchmark 2: $HOME/worktrees/gitoxide/branch-6/target/profiling/gix blame CREDITS
  Time (mean ± σ):      1.591 s ±  0.015 s    [User: 1.411 s, System: 0.174 s]
  Range (min … max):    1.576 s …  1.629 s    10 runs

Summary
  '$HOME/worktrees/gitoxide/branch-6/target/profiling/gix blame CREDITS' ran
    1.39 ± 0.02 times faster than '$HOME/github/Byron/gitoxide/target/profiling/gix blame CREDITS'

[Byron]

🔥🚀❤️