fix(git): serve want-by-sha for unadvertised refs (PR merge commits)

[DanielHabenicht]

Before serving an upload-pack RPC, parse the client's want lines and, for any SHA missing from the mirror, fetch it from upstream on demand and pin it under a reserved refs/proxy-wants/<sha> ref. Pinning makes the object a valid want tip (so upload-pack serves it) and keeps it from git gc. The namespace is hidden from the ref advertisement (uploadpack.hideRefs) yet still honored as a want tip, and excluded from fetch --prune so pins survive periodic refreshes.

Relies on the upstream serving arbitrary SHAs (GitHub's allowAnySHA1InWant); an upstream that refuses leaves the request to fail as before - no regression. Ordinary branch/tag clones pay only a single cheap cat-file check, never an extra upstream call.

What kind of change does this PR introduce?

• fix
• feat
• refactor
• perf
• docs
• test
• build / ci
• chore

Summary

actions/checkout on a pull_request event fetches the synthetic merge commit by bare SHA. That commit lives only under GitHub's unadvertised refs/pull//merge, so clone --mirror never captured it and the mirror's upload-pack rejected the want with "fatal: not our ref ", breaking CI behind the proxy.

Tests

• Added / updated tests (unit, plus integration where it fits)
• Not relevant, because: ...

Checklist

• CI is green locally: cargo fmt --all --check, cargo clippy --all-targets --all-features --locked -- -D warnings, cargo test --all-features
• Commits follow Conventional Commits; AI-assisted commits carry an Assisted-by: trailer (see CONTRIBUTING.md / AGENTS.md)
• Preserves the read-only, pull-only invariant (no push or proactive replication to upstream)
• [-] Docs / README updated if behavior or flags changed

Breaking change?
If yes, describe the impact and the migration path (flags / env, on-disk cache layout).

[codecov]

Codecov Report

❌ Patch coverage is 97.43590% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.80%. Comparing base (9cf2347) to head (996762c).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/git.rs	97.39%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   96.69%   96.80%   +0.10%     
==========================================
  Files           6        6              
  Lines        1846     2065     +219     
==========================================
+ Hits         1785     1999     +214     
- Misses         61       66       +5     

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[rolandjitsu]

Thanks for your contribution @DanielHabenicht ! I left a few comments. You also need to fixup the commit msg/body.

[rolandjitsu]

@DanielHabenicht do you think you'll have the time to address the comments?

[rolandjitsu]

@DanielHabenicht do you think you'll have the time to address the comments?

I'd be happy to pick this up and wrap it up if you're too busy.

[DanielHabenicht]

Sorry, so much todo.
I pushed an update with some of your comments fixed.

You can take it from here.

[rolandjitsu]

Sorry, so much todo. I pushed an update with some of your comments fixed.

You can take it from here.

Thanks for your contributions. No worries. I've taken over and will get this merged in a bit.