Project Diff 2 (#23891)

This adds a new version of the project diff editor to go alongside the
new git panel.

The basics seem to be working, but still todo:

* [ ] Fix untracked files
* [ ] Fix deleted files
* [ ] Show commit message editor at top
* [x] Handle empty state
* [x] Fix panic where locator sometimes seeks to wrong excerpt

Release Notes:

- N/A

crates/go_to_line/src/go_to_line.rs

@@ -80,7 +80,7 @@ impl GoToLine {
             let last_line = editor
                 .buffer()
                 .read(cx)
-                .excerpts_for_buffer(&active_buffer, cx)
+                .excerpts_for_buffer(snapshot.remote_id(), cx)
                 .into_iter()
                 .map(move |(_, range)| text::ToPoint::to_point(&range.context.end, &snapshot).row)
                 .max()