Extend the git API with several new capabilities needed for worktree
archival and restoration:

- Add `allow_empty` flag to `CommitOptions` for creating WIP marker
commits
- Change `create_worktree` to accept `Option<String>` branch, enabling
detached worktree creation when `None` is passed
- Add `head_sha()` to read the current HEAD commit hash
- Add `update_ref()` and `delete_ref()` for managing git references
- Add `stage_all_including_untracked()` to stage everything before a WIP
commit
- Implement all new operations in `FakeGitRepository` with functional
commit history tracking, reset support, and ref management
- Update existing call sites for the new `CommitOptions` field and
`create_worktree` signature

Part 1 of 3 in the persist-worktree stack. These are nonbreaking API
additions with no behavioral changes to existing code.

Release Notes:

- N/A

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Co-authored-by: Anthony Eid <anthony@zed.dev>