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/remote_server/src/headless_project.rs

@@ -200,6 +200,7 @@ impl HeadlessProject {
         client.add_entity_request_handler(Self::handle_stage);
         client.add_entity_request_handler(Self::handle_unstage);
         client.add_entity_request_handler(Self::handle_commit);
+        client.add_entity_request_handler(Self::handle_set_index_text);
         client.add_entity_request_handler(Self::handle_open_commit_message_buffer);
 
         client.add_request_handler(
@@ -691,6 +692,26 @@ impl HeadlessProject {
         Ok(proto::Ack {})
     }
 
+    async fn handle_set_index_text(
+        this: Entity<Self>,
+        envelope: TypedEnvelope<proto::SetIndexText>,
+        mut cx: AsyncApp,
+    ) -> Result<proto::Ack> {
+        let worktree_id = WorktreeId::from_proto(envelope.payload.worktree_id);
+        let work_directory_id = ProjectEntryId::from_proto(envelope.payload.work_directory_id);
+        let repository =
+            Self::repository_for_request(&this, worktree_id, work_directory_id, &mut cx)?;
+        repository
+            .update(&mut cx, |repository, _| {
+                repository.set_index_text(
+                    &RepoPath::from(envelope.payload.path.as_str()),
+                    envelope.payload.text,
+                )
+            })?
+            .await??;
+        Ok(proto::Ack {})
+    }
+
     async fn handle_open_commit_message_buffer(
         this: Entity<Self>,
         envelope: TypedEnvelope<proto::OpenCommitMessageBuffer>,