Use Rope instead of String for buffer diff base (#11300)

As an attempt to do things better when showing diff hunks, store diff
base as Rope, not String, to have cheaper clones when the diff base text
is reused, e.g. creating another buffer with the diff base text for hunk
diff expanding.

Release Notes:

- N/A

crates/git/src/diff.rs

@@ -1,3 +1,4 @@
+use rope::Rope;
 use std::{iter, ops::Range};
 use sum_tree::SumTree;
 use text::{Anchor, BufferId, BufferSnapshot, OffsetRangeExt, Point};
@@ -179,11 +180,12 @@ impl BufferDiff {
         self.tree = SumTree::new();
     }
 
-    pub async fn update(&mut self, diff_base: &str, buffer: &text::BufferSnapshot) {
+    pub async fn update(&mut self, diff_base: &Rope, buffer: &text::BufferSnapshot) {
         let mut tree = SumTree::new();
 
+        let diff_base_text = diff_base.to_string();
         let buffer_text = buffer.as_rope().to_string();
-        let patch = Self::diff(diff_base, &buffer_text);
+        let patch = Self::diff(&diff_base_text, &buffer_text);
 
         if let Some(patch) = patch {
             let mut divergence = 0;
@@ -345,6 +347,7 @@ mod tests {
             three
         "
         .unindent();
+        let diff_base_rope = Rope::from(diff_base.clone());
 
         let buffer_text = "
             one
@@ -355,7 +358,7 @@ mod tests {
 
         let mut buffer = Buffer::new(0, BufferId::new(1).unwrap(), buffer_text);
         let mut diff = BufferDiff::new();
-        smol::block_on(diff.update(&diff_base, &buffer));
+        smol::block_on(diff.update(&diff_base_rope, &buffer));
         assert_hunks(
             diff.hunks(&buffer),
             &buffer,
@@ -364,7 +367,7 @@ mod tests {
         );
 
         buffer.edit([(0..0, "point five\n")]);
-        smol::block_on(diff.update(&diff_base, &buffer));
+        smol::block_on(diff.update(&diff_base_rope, &buffer));
         assert_hunks(
             diff.hunks(&buffer),
             &buffer,
@@ -391,6 +394,7 @@ mod tests {
             ten
         "
         .unindent();
+        let diff_base_rope = Rope::from(diff_base.clone());
 
         let buffer_text = "
             A
@@ -415,7 +419,7 @@ mod tests {
 
         let buffer = Buffer::new(0, BufferId::new(1).unwrap(), buffer_text);
         let mut diff = BufferDiff::new();
-        smol::block_on(diff.update(&diff_base, &buffer));
+        smol::block_on(diff.update(&diff_base_rope, &buffer));
         assert_eq!(diff.hunks(&buffer).count(), 8);
 
         assert_hunks(