fix: Skip post-fetch mirror scan without URL

[pda]

Description

The first no-URL mirror hotfix restored the fast path only when the requested commit was already present. When a canonical fetch added a missing commit, checkout still ran the final global for-each-ref --contains safety scan. On a production mirror that added another 3 minutes 15 seconds after a one-second fetch.

Skip that final reachability scan for the same resolved no-url attempt state. The unpinned-object safeguard remains unchanged whenever a remote mirror URL is configured.

Context

• Production build showing the remaining post-fetch delay
• Follow-up to #4171

Changes

• Reuse the no-URL attempt predicate for both the warm-hit and post-fetch paths.
• Bypass the final global reachability scan only for no-URL attempts.
• Add a PR-ref regression that verifies canonical fetch succeeds without invoking for-each-ref.

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go tool gofumpt -extra -w .)

Focused warm-hit, post-fetch, URL-repair, and configured-remote retry regressions pass under the race detector.

Disclosures / Credits

Cursor Agent correlated the second production trace with the remaining code path, implemented the focused correction and regression coverage, and ran independent ship-risk, maintainability, and simplicity reviews.

  

[cursor]

Rebased and force-pushed the hotfix as d3c28fc3fb49d48774558767af688a4e49dac7f2. The branch is now based directly on 75683492d8057fe3eb0d4ac78ceaed2b1e1930a6; the unrelated 11e51bcf / #4165 merge is no longer in its history. The focused no-URL warm-hit and post-fetch regressions pass on the corrected base.