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/rope/src/point.rs

@@ -1,6 +1,6 @@
 use std::{
     cmp::Ordering,
-    ops::{Add, AddAssign, Sub},
+    ops::{Add, AddAssign, Range, Sub},
 };
 
 /// A zero-indexed point in a text buffer consisting of a row and column.
@@ -20,6 +20,16 @@ impl Point {
         Point { row, column }
     }
 
+    pub fn row_range(range: Range<u32>) -> Range<Self> {
+        Point {
+            row: range.start,
+            column: 0,
+        }..Point {
+            row: range.end,
+            column: 0,
+        }
+    }
+
     pub fn zero() -> Self {
         Point::new(0, 0)
     }