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/outline_panel/src/outline_panel.rs

@@ -1017,7 +1017,7 @@ impl OutlinePanel {
                     .map(|buffer| {
                         active_multi_buffer
                             .read(cx)
-                            .excerpts_for_buffer(&buffer, cx)
+                            .excerpts_for_buffer(buffer.read(cx).remote_id(), cx)
                     })
                     .and_then(|excerpts| {
                         let (excerpt_id, excerpt_range) = excerpts.first()?;