Restructure git diff state management to allow viewing buffers with different diff bases (#21258)

This is a pure refactor of our Git diff state management. Buffers are no
longer are associated with one single diff (the unstaged changes).
Instead, there is an explicit project API for retrieving a buffer's
unstaged changes, and the `Editor` view layer is responsible for
choosing what diff to associate with a buffer.

The reason for this change is that we'll soon want to add multiple "git
diff views" to Zed, one of which will show the *uncommitted* changes for
a buffer. But that view will need to co-exist with other views of the
same buffer, which may want to show the unstaged changes.

### Todo

* [x] Get git gutter and git hunks working with new structure
* [x] Update editor tests to use new APIs
* [x] Update buffer tests
* [x] Restructure remoting/collab protocol
* [x] Update assertions about staged text in
`random_project_collaboration_tests`
* [x] Move buffer tests for git diff management to a new spot, using the
new APIs

Release Notes:

- N/A

---------

Co-authored-by: Richard <richard@zed.dev>
Co-authored-by: Cole <cole@zed.dev>
Co-authored-by: Conrad <conrad@zed.dev>

crates/remote_server/src/remote_editing_tests.rs

@@ -78,13 +78,22 @@ async fn test_basic_remote_editing(cx: &mut TestAppContext, server_cx: &mut Test
         })
         .await
         .unwrap();
+    let change_set = project
+        .update(cx, |project, cx| {
+            project.open_unstaged_changes(buffer.clone(), cx)
+        })
+        .await
+        .unwrap();
+
+    change_set.update(cx, |change_set, cx| {
+        assert_eq!(
+            change_set.base_text_string(cx).unwrap(),
+            "fn one() -> usize { 0 }"
+        );
+    });
 
     buffer.update(cx, |buffer, cx| {
         assert_eq!(buffer.text(), "fn one() -> usize { 1 }");
-        assert_eq!(
-            buffer.diff_base().unwrap().to_string(),
-            "fn one() -> usize { 0 }"
-        );
         let ix = buffer.text().find('1').unwrap();
         buffer.edit([(ix..ix + 1, "100")], None, cx);
     });
@@ -140,9 +149,9 @@ async fn test_basic_remote_editing(cx: &mut TestAppContext, server_cx: &mut Test
         &[(Path::new("src/lib2.rs"), "fn one() -> usize { 100 }".into())],
     );
     cx.executor().run_until_parked();
-    buffer.update(cx, |buffer, _| {
+    change_set.update(cx, |change_set, cx| {
         assert_eq!(
-            buffer.diff_base().unwrap().to_string(),
+            change_set.base_text_string(cx).unwrap(),
             "fn one() -> usize { 100 }"
         );
     });
@@ -213,7 +222,7 @@ async fn test_remote_project_search(cx: &mut TestAppContext, server_cx: &mut Tes
     // test that the headless server is tracking which buffers we have open correctly.
     cx.run_until_parked();
     headless.update(server_cx, |headless, cx| {
-        assert!(!headless.buffer_store.read(cx).shared_buffers().is_empty())
+        assert!(headless.buffer_store.read(cx).has_shared_buffers())
     });
     do_search(&project, cx.clone()).await;
 
@@ -222,7 +231,7 @@ async fn test_remote_project_search(cx: &mut TestAppContext, server_cx: &mut Tes
     });
     cx.run_until_parked();
     headless.update(server_cx, |headless, cx| {
-        assert!(headless.buffer_store.read(cx).shared_buffers().is_empty())
+        assert!(!headless.buffer_store.read(cx).has_shared_buffers())
     });
 
     do_search(&project, cx.clone()).await;