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/text/src/text.rs

@@ -520,8 +520,16 @@ impl Buffer {
     pub fn new(replica_id: u16, remote_id: BufferId, mut base_text: String) -> Buffer {
         let line_ending = LineEnding::detect(&base_text);
         LineEnding::normalize(&mut base_text);
+        Self::new_normalized(replica_id, remote_id, line_ending, Rope::from(base_text))
+    }
 
-        let history = History::new(Rope::from(base_text.as_ref()));
+    pub fn new_normalized(
+        replica_id: u16,
+        remote_id: BufferId,
+        line_ending: LineEnding,
+        normalized: Rope,
+    ) -> Buffer {
+        let history = History::new(normalized);
         let mut fragments = SumTree::new();
         let mut insertions = SumTree::new();