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/git/src/status.rs

@@ -133,6 +133,10 @@ impl FileStatus {
         }
     }
 
+    pub fn has_changes(&self) -> bool {
+        self.is_modified() || self.is_created() || self.is_deleted() || self.is_untracked()
+    }
+
     pub fn is_modified(self) -> bool {
         match self {
             FileStatus::Tracked(tracked) => match (tracked.index_status, tracked.worktree_status) {