Use repository mutex more sparingly. Don't hold it while running git status. (#12489)

Previously, each git `Repository` object was held inside of a mutex.
This was needed because libgit2's Repository object is (as one would
expect) not thread safe. But now, the two longest-running git operations
that Zed performs, (`status` and `blame`) do not use libgit2 - they
invoke the `git` executable. For these operations, it's not necessary to
hold a lock on the repository.

In this PR, I've moved our mutex usage so that it only wraps the libgit2
calls, not our `git` subprocess spawns. The main user-facing impact of
this is that the UI is much more responsive when initially opening a
project with a very large git repository (e.g. `chromium`, `webkit`,
`linux`).

Release Notes:

- Improved Zed's responsiveness when initially opening a project
containing a very large git repository.

crates/vcs_menu/src/lib.rs

@@ -109,7 +109,7 @@ impl BranchListDelegate {
             .get_first_worktree_root_repo(cx)
             .context("failed to get root repository for first worktree")?;
 
-        let all_branches = repo.lock().branches()?;
+        let all_branches = repo.branches()?;
         Ok(Self {
             matches: vec![],
             workspace: handle,
@@ -237,7 +237,6 @@ impl PickerDelegate for BranchListDelegate {
                         .get_first_worktree_root_repo(cx)
                         .context("failed to get root repository for first worktree")?;
                     let status = repo
-                        .lock()
                         .change_branch(&current_pick);
                     if status.is_err() {
                         this.delegate.display_error_toast(format!("Failed to checkout branch '{current_pick}', check for conflicts or unstashed files"), cx);
@@ -316,8 +315,6 @@ impl PickerDelegate for BranchListDelegate {
                                             let repo = project
                                                 .get_first_worktree_root_repo(cx)
                                                 .context("failed to get root repository for first worktree")?;
-                                            let repo = repo
-                                                .lock();
                                             let status = repo
                                                 .create_branch(&current_pick);
                                             if status.is_err() {