Improves merge-conflict loading by resolving ordered conflict anchors
with a single excerpt sweep instead of restarting the excerpt scan four
times per conflict.

The pull request also adds `script/generate-merge-conflict-benchmark`,
which creates validated small, medium, or severe repositories with real
unresolved Git merges.

On the severe workload (128 files × 256 conflicts), anchor conversion
decreased from 483.5 ms to 5.9 ms, saving about 478 ms, or roughly 5% of
the total refresh.

Closes
https://linear.app/zed-industries/issue/FR-174/improve-git-panel-performance-for-large-merge-conflicts

## Testing

- `cargo test -p multi_buffer
test_ordered_buffer_anchor_ranges_to_anchor_ranges`
- `cargo test -p git_ui conflict`
- `./script/clippy -p multi_buffer -p git_ui`
- Generated and validated the small synthetic merge-conflict workload

Release Notes:

- Improved performance when opening large merge conflicts