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/project/src/project.rs

@@ -610,6 +610,7 @@ impl Project {
         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);
 
         WorktreeStore::init(&client);
@@ -4092,6 +4093,27 @@ impl Project {
         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_handle =
+            Self::repository_for_request(&this, worktree_id, work_directory_id, &mut cx)?;
+
+        repository_handle
+            .update(&mut cx, |repository_handle, _| {
+                repository_handle.set_index_text(
+                    &RepoPath::from_str(&envelope.payload.path),
+                    envelope.payload.text,
+                )
+            })?
+            .await??;
+        Ok(proto::Ack {})
+    }
+
     async fn handle_open_commit_message_buffer(
         this: Entity<Self>,
         envelope: TypedEnvelope<proto::OpenCommitMessageBuffer>,
@@ -4336,6 +4358,27 @@ impl Project {
     pub fn all_repositories(&self, cx: &App) -> Vec<Entity<Repository>> {
         self.git_state.read(cx).all_repositories()
     }
+
+    pub fn repository_and_path_for_buffer_id(
+        &self,
+        buffer_id: BufferId,
+        cx: &App,
+    ) -> Option<(Entity<Repository>, RepoPath)> {
+        let path = self
+            .buffer_for_id(buffer_id, cx)?
+            .read(cx)
+            .project_path(cx)?;
+        self.git_state
+            .read(cx)
+            .all_repositories()
+            .into_iter()
+            .find_map(|repo| {
+                Some((
+                    repo.clone(),
+                    repo.read(cx).repository_entry.relativize(&path.path).ok()?,
+                ))
+            })
+    }
 }
 
 fn deserialize_code_actions(code_actions: &HashMap<String, bool>) -> Vec<lsp::CodeActionKind> {