git: Compute and synchronize diffs from HEAD (#23626)

This PR builds on #21258 to make it possible to use HEAD as a diff base.
The buffer store is extended to support holding multiple change sets,
and collab gains support for synchronizing the committed text of files
when any collaborator requires it.

Not implemented in this PR:

- Exposing the diff from HEAD to the user
- Decorating the diff from HEAD with information about which hunks are
staged

`test_random_multibuffer` now fails first at `SEED=13277`, similar to
the previous high-water mark, but with various bugs in the multibuffer
logic now shaken out.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Ben <ben@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>
Co-authored-by: Conrad <conrad@zed.dev>

crates/sum_tree/src/sum_tree.rs

@@ -516,6 +516,10 @@ impl<T: Item> SumTree<T> {
         }
     }
 
+    pub fn ptr_eq(&self, other: &Self) -> bool {
+        Arc::ptr_eq(&self.0, &other.0)
+    }
+
     fn push_tree_recursive(
         &mut self,
         other: SumTree<T>,