Support opening bare repositories

[mjc]

Summary

This lets guitar open bare repositories for history browsing without a working tree. Repository loads now derive worktree capability from the currently opened git2::Repository, so reloading or switching repositories cannot leave stale bare/worktree state behind.

The same change keeps workdir status collection on libgit2's recursive status entries, rather than expanding untracked directories through filesystem paths and re-querying each child. That avoids path re-resolution problems on case-insensitive filesystems when a repo contains case-colliding filenames from another platform.

A better way to do this in the future would be to move toward more of a state machine than a bunch of bools.

Details

• Detect missing worktrees from the opened repository instead of a sticky app-level startup mode.
• Model command availability through Command::required_capability() and current repository worktree support.
• Treat repositories without a worktree as having no workdir status result at the query boundary, then show an empty uncommitted-change view.
• Keep Git identity loading independent of worktree availability.
• Block worktree-mutating actions when the current repository has no worktree.
• Preserve conflict and submodule status handling while using recursive status entries for untracked directories.

Details

• Detect bare repositories from the opened repository instead of a sticky app-level mode flag.
• Model command availability through Command::required_capability() and current repository worktree support.
• Treat repositories without a worktree as having no uncommitted-change result at the query boundary, so graph/history browsing can load cleanly.
• Keep Git identity loading independent of worktree availability.
• Block worktree-mutating actions when the current repository has no worktree.
• Preserve conflict and submodule status handling while using recursive status entries for untracked directories.

Tests

• cargo test bare_repo -- --nocapture
• cargo test unavailable_worktree_blocks_worktree_commands_before_dispatch -- --nocapture
• cargo test reload_refreshes_worktree_state_for_new_repository -- --nocapture
• cargo test workdir_diff -- --nocapture