commit squash: use merge arithmetic

[estib-vega]

Use merge arithmetic instead of reordering the commits for a faster,
more reliable squash.

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This is part 2 of 3 in a stack made with GitButler:

•  3  graphy remote branch integration #13881
•  2  commit squash: use merge arithmetic #13873 👈
•  1  editor: merge commit changes into tree #13765

[Copilot]

Pull request overview

This PR updates the workspace commit-squash implementation to compute the squashed tree via merge arithmetic (merging only the selected commits’ change ranges) rather than reordering commits around the target, aiming for faster and more reliable squashes.

Changes:

• Replaces the “reorder then squash” approach with merge_commit_changes_to_tree to derive the squashed tree and fails early on merge conflicts.
• Updates an existing squash-conflict test to assert the new error behavior/message.
• Adds a new workspace scenario + test ensuring cross-stack squashes don’t accidentally pull in unselected ancestor tree state.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
crates/but-workspace/src/commit/squash_commits.rs	Switches squash to merge-arithmetic tree construction, removes reordering logic, and rewrites graph steps to drop subject commits and replace the target.
crates/but-workspace/tests/workspace/commit/squash_commits.rs	Updates conflict expectation and adds a regression test for cross-stack squashes not pulling in unselected ancestor state.
crates/but-workspace/tests/fixtures/scenario/ws-ref-ws-commit-single-stack-double-stack-files.sh	Adds a new fixture repository topology used by the new regression test.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.