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/editor_tests.rs

@@ -14047,6 +14047,59 @@ async fn test_edit_after_expanded_modification_hunk(
     );
 }
 
+#[gpui::test]
+async fn test_stage_and_unstage_added_file_hunk(
+    executor: BackgroundExecutor,
+    cx: &mut gpui::TestAppContext,
+) {
+    init_test(cx, |_| {});
+
+    let mut cx = EditorTestContext::new(cx).await;
+    cx.update_editor(|editor, _, cx| {
+        editor.set_expand_all_diff_hunks(cx);
+    });
+
+    let working_copy = r#"
+            ˇfn main() {
+                println!("hello, world!");
+            }
+        "#
+    .unindent();
+
+    cx.set_state(&working_copy);
+    executor.run_until_parked();
+
+    cx.assert_state_with_diff(
+        r#"
+            + ˇfn main() {
+            +     println!("hello, world!");
+            + }
+        "#
+        .unindent(),
+    );
+    cx.assert_index_text(None);
+
+    cx.update_editor(|editor, window, cx| {
+        editor.toggle_staged_selected_diff_hunks(&ToggleStagedSelectedDiffHunks, window, cx);
+    });
+    executor.run_until_parked();
+    cx.assert_index_text(Some(&working_copy.replace("ˇ", "")));
+    cx.assert_state_with_diff(
+        r#"
+            + ˇfn main() {
+            +     println!("hello, world!");
+            + }
+        "#
+        .unindent(),
+    );
+
+    cx.update_editor(|editor, window, cx| {
+        editor.toggle_staged_selected_diff_hunks(&ToggleStagedSelectedDiffHunks, window, cx);
+    });
+    executor.run_until_parked();
+    cx.assert_index_text(None);
+}
+
 async fn setup_indent_guides_editor(
     text: &str,
     cx: &mut gpui::TestAppContext,