Correct octopus merge conflict message to show both tree IDs

[Copilot]

In the 2-parent merge conflict path of octopus() (crates/but-rebase/src/merge.rs), the error message printed the same tree hash twice, which obscured which side actually conflicted. This updates the message construction so it reports the conflicting tree and the previously merged tree distinctly.

• Conflict detail formatting

  • Extracted message suffix construction into merge_conflict_details(...).
  • Updated the 2-parent branch to reference the first successfully merged tree (ours) instead of reusing tree_to_merge.
  • Kept multi-parent/octopus formatting behavior intact (list all successfully merged trees).

• Defensive behavior

  • Added an explicit defensive empty-slice behavior in merge_conflict_details (empty suffix), with inline context comment.

• Focused unit coverage

  • Added tests for:
    • 2-parent case (single successfully merged tree),
    • multi-parent case (list of merged trees),
    • defensive empty-input case.

// before (2-parent conflict path):
format!(" and tree {tree_to_merge}") // duplicated hash in final message

// now:
format!(" and tree {}", successfully_merged[0]) // shows the other side

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
gitbutler-web	Ignored	Preview	Mar 12, 2026 5:21am

[Byron]

While this was only an armchair review, I think it's good. The tests cement the desired behaviour as well, and since we had a bug there it's fair to keep them.

[Copilot]

Pull request overview

Refactors the octopus-merge conflict error message formatting in but-rebase by extracting the “previously merged trees” suffix logic into a helper and adding unit tests to pin the output.

Changes:

• Extracted conflict-details formatting into merge_conflict_details().
• Updated the octopus merge conflict error to use the new helper.
• Added unit tests covering 0/1/2 successfully-merged-tree cases.

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