git: Fix race condition when [un]staging hunks in quick succession (#26422)

- [x] Fix `[un]stage` hunk operations cancelling pending ones
  - [x] Add test
- [ ] bugs I stumbled upon (try to repro again before merging)
  - [x] holding `git::StageAndNext` skips hunks randomly 
    - [x] Add test
  - [x] restoring a file keeps it in the git panel
- [x] Double clicking on `toggle staged` fast makes Zed disagree with
`git` CLI
- [x] checkbox shows ✔️ (fully staged) after a single
stage

Release Notes:

- N/A

---------

Co-authored-by: Cole <cole@zed.dev>
Co-authored-by: Max <max@zed.dev>

crates/sum_tree/src/cursor.rs

@@ -406,6 +406,7 @@ where
         self.seek_internal(pos, bias, &mut (), cx)
     }
 
+    /// Advances the cursor and returns traversed items as a tree.
     #[track_caller]
     pub fn slice<Target>(
         &mut self,