project: Fix branch diff with older servers (#62134)

# Objective

Prevent branch diffs from failing when a server does not recognize
worktree diff requests.

## Solution

Send `HEAD` as a valid committed-only fallback for servers that ignore
`includes_worktree`.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [ ] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

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Release Notes:

- N/A
41c0f28bf9Ben Kunkle committed on 8/3/2026, 8:24:39 PM· committed by GitHubparent7759e9f
1 file changedLine totals unavailable