ci: the picker fetches kept bases' trees, not just checks their commits

[teamleaderleo]

Why

Even after #15040, 77 of 240 distance-picker candidates from 18:16-18:45Z were unknown. The recorded fetch reports show a pattern: bases compared = bases fetched + 1. On #15003's run, 2 of 15 compared with missing: 0, so nothing was fetched. The bases the picker didn't fetch never diffed.

Cause: the changes job's delta_since_green.py fetches main's history with --filter=tree:0. Most kept merge bases are recent main commits, so they are already in the checkout as commits without trees. fetch_bases() checked only have_commit and skipped them, git diff then failed (no lazy fetch), and every such root cost the unknown start.

Change

• have_tree() checks the commit's root tree.
• fetch_bases() uses have_tree() and fetches with --refetch, so the server sends the trees of commits the checkout already holds. It's still depth 1 and blobless.

Check

I replayed it on a fresh depth-2 checkout of #15013's merge commit, plus the same tree:0 --deepen history, against the 17:29Z janitor snapshot:

• before: 1 of 19 bases diffable
• after: 19 of 19, with a 0.8 s fetch

Tests: test_a_commit_without_its_tree_counts_as_missing, and the fetch test now asserts --refetch. Targeted -k runs pass locally.

🤖 Generated with Claude Code

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Summary by cubic

Fixes the distance picker marking candidates as unknown when kept merge bases are already present as commits without trees (from --filter=tree:0 fetches). The picker now checks for the commit's root tree and fetches with --refetch, so the server sends the missing trees and all kept bases can be diffed.

^{Written for commit 8a3f017. Summary will update on new commits.}

[github-actions]

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[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 2570eed and 8a3f017.

📒 Files selected for processing (2)

• scripts/ci/warm_distance.py
• tests/test_ci_warm_distance.py

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[teamleaderleo]

Review note: --refetch --depth=1 on a base whose bare commit is already present adds that base to .git/shallow, which cuts the history behind it. The picker runs after delta_since_green.py, and the later steps in the changes job read only HEAD and HEAD^1 (the job's own base is excluded from the refetch), so nothing reads that history today. A future history walk placed after the picker in that job would have to deepen first.

[github-actions]

Merge receipt for 8a3f01701f: every check was green at merge (13 verified; 18 skipped by policy). Full suite runs on main after merge.