feat: optimize detached HEAD merge base for jj users

[121watts]

Summary

For detached-HEAD workflows (standard jj/Jujutsu pattern), findMergeBase now finds the closest pushed ancestor commit instead of falling all the way back to origin/main. This produces smaller, more precise patches when running depot ci run with local changes.

What was happening

When HEAD is detached, findMergeBase skipped the remote tracking branch check entirely (since there's no named branch) and fell back to git merge-base HEAD origin/main. For jj users who always work in detached HEAD but have pushed branches, this meant the patch included the entire branch diff from main -- potentially thousands of lines -- instead of just the unpushed local changes.

What happens now

When HEAD is detached, findClosestRemoteAncestor enumerates all refs under refs/remotes/origin/, checks which ones are ancestors of HEAD, and picks the one with the shortest distance (fewest commits between it and HEAD). This finds the most recent pushed commit that's an ancestor of the current working state.

Falls back to the existing default-branch merge base behavior if no remote tracking ref is an ancestor of HEAD.

Anything else?

This is a pure optimization -- the worst case is identical to current behavior (falls back to origin/main). Edge cases with jj's ref model (bookmarks vs working copy) should be safe since we're only reading standard git refs. Tested with a simulated jj-style workflow: push branch, add local commits, detach HEAD.

Closes DEP-3974

Made with Cursor

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Note

Medium Risk
Medium risk: changes how depot ci run computes the merge base for patch generation in detached-HEAD states, which could alter patch contents if the wrong remote ancestor is selected; fallback behavior remains unchanged when no ancestor is found.

Overview
Improves patch generation for detached-HEAD workflows by teaching findMergeBase to prefer the closest pushed origin/* ancestor commit instead of defaulting straight to the default branch merge base.

Adds findClosestRemoteAncestor, which scans refs/remotes/origin/*, filters to ancestors of HEAD, and selects the one with the fewest commits to HEAD, producing smaller patches for jj-style workflows. Includes a new unit test covering the detached-HEAD-with-pushed-ancestor scenario.

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[linear]

DEP-3974 CLI: Optimize detached HEAD merge base for jj users