Add the ability to propose changes to a set of buffers (#18170)

This PR introduces functionality for creating *branches* of buffers that
can be used to preview and edit change sets that haven't yet been
applied to the buffers themselves.

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Marshall <marshall@zed.dev>

crates/text/src/text.rs

@@ -13,6 +13,7 @@ mod undo_map;
 pub use anchor::*;
 use anyhow::{anyhow, Context as _, Result};
 pub use clock::ReplicaId;
+use clock::LOCAL_BRANCH_REPLICA_ID;
 use collections::{HashMap, HashSet};
 use locator::Locator;
 use operation_queue::OperationQueue;
@@ -715,6 +716,19 @@ impl Buffer {
         self.snapshot.clone()
     }
 
+    pub fn branch(&self) -> Self {
+        Self {
+            snapshot: self.snapshot.clone(),
+            history: History::new(self.base_text().clone()),
+            deferred_ops: OperationQueue::new(),
+            deferred_replicas: HashSet::default(),
+            lamport_clock: clock::Lamport::new(LOCAL_BRANCH_REPLICA_ID),
+            subscriptions: Default::default(),
+            edit_id_resolvers: Default::default(),
+            wait_for_version_txs: Default::default(),
+        }
+    }
+
     pub fn replica_id(&self) -> ReplicaId {
         self.lamport_clock.replica_id
     }