Move repository state RPC handlers to the GitStore (#27391)

This is another in the series of PRs to make the GitStore own all
repository state and enable better concurrency control for git
repository scans.

After this PR, the `RepositoryEntry`s stored in worktree snapshots are
used only as a staging ground for local GitStores to pull from after
git-related events; non-local worktrees don't store them at all,
although this is not reflected in the types. GitTraversal and other
places that need information about repositories get it from the
GitStore. The GitStore also takes over handling of the new
UpdateRepository and RemoveRepository messages. However, repositories
are still discovered and scanned on a per-worktree basis, and we're
still identifying them by the (worktree-specific) project entry ID of
their working directory.

- [x] Remove WorkDirectory from RepositoryEntry
- [x] Remove worktree IDs from repository-related RPC messages
- [x] Handle UpdateRepository and RemoveRepository RPCs from the
GitStore

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/git_ui/src/project_diff.rs

@@ -343,7 +343,8 @@ impl ProjectDiff {
                 if !entry.status.has_changes() {
                     continue;
                 }
-                let Some(project_path) = repo.repo_path_to_project_path(&entry.repo_path) else {
+                let Some(project_path) = repo.repo_path_to_project_path(&entry.repo_path, cx)
+                else {
                     continue;
                 };
                 let namespace = if repo.has_conflict(&entry.repo_path) {