Add tests for Myers false-positive merge conflicts

Reproduce the gitoxide Myers diff bug (GitoxideLabs/gitoxide#2475)
in the context of GitButler workspace management. The bug causes
non-overlapping edits to the same file to be reported as conflicts,
which leads to branches being falsely unapplied from the workspace.

Test fixtures (shell scenarios):
- myers-false-conflict-same-file.sh: two branches each deleting a
  different line from a shared file (delete-alpha, delete-bravo).
- myers-false-conflict-multi-commit.sh: same pattern but with two
  sequential commits per branch, for squash/amend coverage.

Test cases (myers_false_conflict.rs):
- Basic merge: both stacks should merge cleanly, but the second
  branch is falsely unapplied.
- Hero stack: hero is preserved, non-hero is falsely unapplied.
- Multi-commit branches: same false unapply with deeper history.
- Squash then merge: squash succeeds but subsequent workspace merge
  still falsely unapplies the other branch.
- Amend then merge: amend succeeds but workspace merge still falsely
  unapplies the other branch.
- Stack ordering: the false conflict is symmetric — the second stack
  is always the one unapplied regardless of order.

All five conflict-related tests currently fail against unpatched
gitoxide and should pass once the upstream fix lands.

Author: mtsgrd

crates/but-workspace/tests/fixtures/scenario/myers-false-conflict-multi-commit.sh

@@ -0,0 +1,62 @@
+#!/usr/bin/env bash
+
+set -eu -o pipefail
+source "${BASH_SOURCE[0]%/*}/shared.sh"
+
+### Description
+# Similar to myers-false-conflict-same-file but with multiple sequential commits
+# per branch, to test squash and amend operations in the context of false conflicts.
+#
+# Base file:
+#   alpha_x
+#   (blank)
+#   bravo_x
+#   charlie_x
+#   (blank)
+#
+# Branch "edit-alpha" has two commits:
+#   1. Adds a comment line at the top
+#   2. Deletes alpha_x (replaces with blank) and removes trailing blank
+#
+# Branch "edit-bravo" has two commits:
+#   1. Adds a comment line at the bottom
+#   2. Deletes bravo_x
+#
+# The final state of each branch triggers the Myers false conflict.
+# Squashing the two commits in either branch should also work but may
+# trigger merge issues during the rebase.
+
+git init
+
+# Create the base content on main
+printf 'alpha_x\n\nbravo_x\ncharlie_x\n\n' > shared-file
+git add shared-file
+git commit -m "base: shared-file with alpha, bravo, charlie"
+
+git branch gitbutler/workspace
+
+# Branch A: two sequential commits
+git checkout -b edit-alpha main
+
+# First commit: add a header comment (doesn't conflict with anything)
+printf '# header\nalpha_x\n\nbravo_x\ncharlie_x\n\n' > shared-file
+git add shared-file
+git commit -m "add header to shared-file"
+
+# Second commit: delete alpha_x and trailing blank
+printf '# header\n\n\nbravo_x\ncharlie_x\n' > shared-file
+git add shared-file
+git commit -m "delete alpha_x from shared-file"
+
+# Branch B: two sequential commits
+git checkout -b edit-bravo main
+
+# First commit: add a footer comment (doesn't conflict with anything)
+printf 'alpha_x\n\nbravo_x\ncharlie_x\n\n# footer\n' > shared-file
+git add shared-file
+git commit -m "add footer to shared-file"
+
+# Second commit: delete bravo_x
+printf 'alpha_x\n\ncharlie_x\n\n# footer\n' > shared-file
+git add shared-file
+git commit -m "delete bravo_x from shared-file"

crates/but-workspace/tests/fixtures/scenario/myers-false-conflict-same-file.sh

@@ -0,0 +1,43 @@
+#!/usr/bin/env bash
+
+set -eu -o pipefail
+source "${BASH_SOURCE[0]%/*}/shared.sh"
+
+### Description
+# Two independent branches editing the same file with non-overlapping changes.
+# This triggers a false merge conflict with Myers diff algorithm due to
+# split hunks with empty insertions.
+# See: https://github.com/GitoxideLabs/gitoxide/issues/2475
+#
+# Base file has:
+#   alpha_x
+#   (blank)
+#   bravo_x
+#   charlie_x
+#   (blank)
+#
+# Branch "delete-alpha" removes alpha_x (replaces with blank) and trailing blank.
+# Branch "delete-bravo" removes bravo_x.
+# These are non-overlapping edits that git merge-file resolves cleanly,
+# but Myers diff produces a false conflict.
+
+git init
+
+# Create the base content on main
+printf 'alpha_x\n\nbravo_x\ncharlie_x\n\n' > shared-file
+git add shared-file
+git commit -m "base: shared-file with alpha, bravo, charlie"
+
+git branch gitbutler/workspace
+
+# Branch A: delete alpha_x (replace with blank line) and remove trailing blank
+git checkout -b delete-alpha main
+printf '\n\nbravo_x\ncharlie_x\n' > shared-file
+git add shared-file
+git commit -m "delete alpha_x from shared-file"
+
+# Branch B: delete bravo_x
+git checkout -b delete-bravo main
+printf 'alpha_x\n\ncharlie_x\n\n' > shared-file
+git add shared-file
+git commit -m "delete bravo_x from shared-file"

crates/but-workspace/tests/workspace/commit/mod.rs

@@ -6,6 +6,8 @@ mod move_commit;
 mod reword;
 mod uncommit_changes;
 
+mod myers_false_conflict;
+
 mod from_new_merge_with_metadata {
     use bstr::ByteSlice;
     use but_graph::init::{Options, Overlay};