docs: show how to benchmark across git branches

[xfocus3]

Summary

• Add a README example for benchmarking the same command across Git branches.
• Use --parameter-list with --setup 'git switch {branch}' so branch switching is outside the timed command.
• Mention git worktree for benchmarking uncommitted changes.

Closes #606.

Verification

• Confirmed in src/benchmark/mod.rs that --setup receives parameter substitution and runs before the set of timing runs.
• Ran git diff --check.

[ded-furby]

Nice example. One side effect that may be worth calling out explicitly in the surrounding text: after the benchmark finishes, the repo will still be left on the last branch from because mutates checkout state outside the timed command.\n\nSince the paragraph already warns about a clean working tree, an extra sentence about ending on the final branch would help avoid surprising users who run this in their main checkout instead of a disposable worktree.

[ded-furby]

Nice example. One side effect that may be worth calling out explicitly in the surrounding text: after the benchmark finishes, the repo will still be left on the last branch from --parameter-list because --setup 'git switch {branch}' mutates checkout state outside the timed command.

Since the paragraph already warns about a clean working tree, an extra sentence about ending on the final branch would help avoid surprising users who run this in their main checkout instead of a disposable worktree.