Fix workspace merge failure when stacks have adjacent edits

[mtsgrd]

When two stacks each modify adjacent but non-overlapping lines in the
same file, git2 treats the adjacent hunks as conflicting. This caused
update_uncommitted_changes to fail with MergeConflict (-24) on any
mutation that recomputed the workspace tree (squash, reorder, etc.)
— even though the stacks don't actually conflict.

The workspace commit was already built using gix
(remerged_workspace_tree_v2), which resolves adjacent hunks cleanly,
so it was possible to reach a valid workspace state that then broke on
every subsequent operation.

Fix

Replace the git2 octopus merge in merge_workspace with the same
gix-based approach used for workspace commit creation:
merge_options_fail_fast + has_unresolved_conflicts. The function
now returns gix::ObjectId; callers in integrate.rs that pass the
result into git2 APIs convert with .to_git2().

Test

Add a regression test that calls merge_workspace directly with two
stacks whose edits are adjacent from both sides (Stack A owns lines 1–5
and 11–15, Stack B owns lines 6–10). Verified the test fails with
MergeConflict (-24) under git2 and passes under gix.

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
gitbutler-web	Ignored	Preview	Apr 2, 2026 11:36am

[Copilot]

Pull request overview

This PR fixes false-positive merge conflicts during workspace mutations (e.g., squash/reorder) caused by a divergence between git2’s Myers diff merge behavior and gix’s merge behavior when stacks modify adjacent but non-overlapping hunks. It aligns workspace tree computation with the already-used gix merge approach so valid workspace states don’t later fail during recomputation.

Changes:

• Replaced the git2-based octopus merge in merge_workspace with a gix-based merge using merge_options_fail_fast + has_unresolved_conflicts.
• Updated workspace tree recomputation paths to use a merge-optimized gix::Repository, converting results to git2 OIDs only at git2 API boundaries.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
crates/gitbutler-workspace/src/branch_trees.rs	Switches workspace tree octopus merge from git2 to gix and threads a merge-optimized gix repo through uncommitted-change update flows.
crates/but-worktrees/src/integrate.rs	Updates integration conflict checks to compute workspace trees via gix and convert to git2 only when calling move_tree.

[mtsgrd]

@Byron this is different from the others but it has a similar consequence. In this screenshot there are two commits that touch Segment.svelte, they do not overlap, but have adjacent edits on lines 24:25.

As a result, any operation that recomputes the workspace commit failed, and knowing why takes knowledge of this bug. It took me a few minutes to find the culprit, even though I knew what to look for.

command: squash_commits
params: {"projectId":"L1VzZXJzL21hdHRpYXMvYy9naXRidXRsZXIvLmdpdA","stackId":"5323a6a0-fbcf-41ad-987e-9344d96a9a99","sourceCommitIds":["b8460f75257ee8dc5e5b9cf54caccf409892bc75"],"targetCommitId":"1ffae114a3d46b6e547c62b92a2e527a18829819"})

merge conflicts exist; class=Merge (22); code=MergeConflict (-24)

[Byron]

It's good to see that gix is actually fixing something merge-related this time 😅.