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/git_ui/src/project_diff.rs

@@ -278,7 +278,7 @@ impl ProjectDiff {
                     has_staged_hunks = true;
                     has_unstaged_hunks = true;
                 }
-                DiffHunkSecondaryStatus::None
+                DiffHunkSecondaryStatus::NoSecondaryHunk
                 | DiffHunkSecondaryStatus::SecondaryHunkRemovalPending => {
                     has_staged_hunks = true;
                 }