docs: make blame.ignoreRevsFile discoverable and automatic

[dlovell]

Problem

The repo has a .git-blame-ignore-revs file, but it does nothing in a fresh clone. Git deliberately refuses to read blame.ignoreRevsFile from repository-tracked config (that would let a repo change local git behavior), so every contributor has to set it by hand — and nothing in the repo says so. GitHub's web blame reads the file automatically, so the local/web mismatch is easy to miss.

Changes

• CONTRIBUTING.md — document the one-liner (git config --replace-all blame.ignoreRevsFile .git-blame-ignore-revs) right after the existing pre-commit install step, and a "git blame and bulk reformats" subsection covering how the hook sets it, that existing clones must re-run pre-commit install to pick up the new hook type, and the caveat below. This is the one place any of it is written down.
• .git-blame-ignore-revs — a three-line header saying what the file is and pointing at that subsection, rather than restating it in a file that is mostly machine-read.
• .pre-commit-config.yaml — a post-checkout hook that sets the config, plus default_install_hook_types: [pre-commit, post-checkout] so pre-commit install wires both. Contributors who follow the setup steps get it without running the extra command.

--replace-all rather than plain git config <name> <value>: the single-value form hard-fails (exit 5, cannot overwrite multiple values with a single value) when a clone's local config already holds more than one blame.ignoreRevsFile entry, which git explicitly supports. The hook would then print Failed on every checkout with no way to recover, since each retry hits the same error. --replace-all collapses the list to the one correct value.

Verification

In a fresh clone of this branch (pre-commit 4.6.0, git 2.55):

• blame.ignoreRevsFile starts unset.
• pre-commit install reports installed at .git/hooks/pre-commit and installed at .git/hooks/post-checkout.
• The next checkout runs configure blame.ignoreRevsFile...Passed, after which git config --get blame.ignoreRevsFile returns .git-blame-ignore-revs.
• Seeded with two local blame.ignoreRevsFile values, the hook still passes and leaves exactly one value. (With the single-value form it exited 5 and failed on every subsequent checkout.)
• An existing post-checkout hook (e.g. git-lfs) is preserved — pre-commit migrates it via its .legacy mechanism.

Caveats

• git blame fails at revs predating this file. Once the config is set, git blame on any file exits 128 with fatal: could not open object name list: .git-blame-ignore-revs whenever the working tree is at a rev older than 14a30475 (2024-05-17) — bisecting into early history, or checking out a tag up to v0.1.2.post. There is no per-command override: blame.ignoreRevsFile is multi-valued, so -c blame.ignoreRevsFile= and --ignore-revs-file=""/=/dev/null all append rather than replace, and blame still tries to open the configured path. The workaround is git config --unset-all blame.ignoreRevsFile for the duration; the next checkout restores it. This is documented in CONTRIBUTING.md.
• No visible blame difference today. The single rev currently listed (01b9b5aa, the ruff import-order pass) no longer owns any lines in any file. This PR is about the mechanism being in place and correct for the next bulk reformat, not about fixing blame output right now.

Notes

• default_install_hook_types changes what pre-commit install installs, so existing clones need to re-run pre-commit install once to pick up the post-checkout hook. New clones get it from the documented setup flow.
• The hook is stages: [post-checkout] only, so it never runs on git commit and pre-commit.ci is unaffected (it doesn't run post-checkout hooks). It does run on file checkouts, though: git checkout -- <file> measured 0.153s with the hook vs 0.004s without, and git restore <file> prints a configure blame.ignoreRevsFile...Passed line each time.
• Branch is named docs-blame-ignore-revs rather than docs/blame-ignore-revs: origin already has a branch literally named docs, which blocks any docs/* ref.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main    #2275      +/-   ##
==========================================
- Coverage   83.35%   83.34%   -0.01%     
==========================================
  Files         165      165              
  Lines       21534    21534              
==========================================
- Hits        17949    17947       -2     
- Misses       3585     3587       +2     

Flag	Coverage Δ
catalog-annex	49.42% <ø> (ø)
catalog-git	54.50% <ø> (ø)
core	67.44% <ø> (-0.01%)	⬇️
library	0.43% <ø> (ø)
slow	52.59% <ø> (ø)

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see 1 file with indirect coverage changes

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