Worktree support: Phase 1

[Uncommon]

Recognize worktrees and treat them on par with the main worktree. No management features yet.

• [ ] Use git_repository_item_path for finding files in the git directory.
• [ ] In the sidebar, call the Staging section Worktrees and list all worktrees in it, with the main one called "Main". Use the shortest unique path name for others.
• [ ] Show each worktree's active branch next to its name (only when there are linked worktrees?)
• [ ] When a worktree is selected, highlight its active branch in the sidebar branch list.
• [ ] In the history list, any branch checked out by a worktree should be highlighted.
• [ ] Indicate locked status for each worktree.
• [ ] Watch worktree repositories for workspace changes.
• [ ] Show staging appropriate to the selected worktree.