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 :heavy_check_mark: (fully staged) after a single
stage

Release Notes:

- N/A

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