Implement staging and unstaging hunks (#24606)

- [x] Staging hunks
- [x] Unstaging hunks
- [x] Write a randomized test
- [x] Get test passing
- [x] Fix existing bug in diff_base_byte_range computation
- [x] Remote project support
- [ ] ~~Improve performance of
buffer_range_to_unchanged_diff_base_range~~
- [ ] ~~Bug: project diff editor scrolls to top when staging/unstaging
hunk~~ existing issue
- [ ] ~~UI~~ deferred
- [x] Tricky cases
  - [x] Correctly handle acting on multiple hunks for a single file
- [x] Remove path from index when unstaging the last staged hunk, if
it's absent from HEAD, or staging the only hunk, if it's deleted in the
working copy

Release Notes:

- Add `ToggleStagedSelectedDiffHunks` action for staging and unstaging
individual diff hunks

crates/editor/src/test/editor_test_context.rs

@@ -298,6 +298,18 @@ impl EditorTestContext {
         self.cx.run_until_parked();
     }
 
+    pub fn assert_index_text(&mut self, expected: Option<&str>) {
+        let fs = self.update_editor(|editor, _, cx| {
+            editor.project.as_ref().unwrap().read(cx).fs().as_fake()
+        });
+        let path = self.update_buffer(|buffer, _| buffer.file().unwrap().path().clone());
+        let mut found = None;
+        fs.with_git_state(&Self::root_path().join(".git"), false, |git_state| {
+            found = git_state.index_contents.get(path.as_ref()).cloned();
+        });
+        assert_eq!(expected, found.as_deref());
+    }
+
     /// Change the editor's text and selections using a string containing
     /// embedded range markers that represent the ranges and directions of
     /// each selection.