Fix the project diff sometimes missing updates (#40662)

This PR does two related things:

- First, it gets rid of the undifferentiated `RepositoryEvent::Updated`
in favor of three new events that have clearer definitions:
`BranchChanged`, `StashEntriesChanged`, and `StatusesChanged`. An
implication of this is that we no longer emit a `RepositoryEvent` unless
some git state changed; previously we would emit `RepositoryUpdated`
after doing a git status scan even if no statuses changed.
- Second, it changes the subscription strategy of the project diff to
make it update more robustly. Previously, the project diff only
subscribed to the `GitStore`, so it relied on getting a `GitStoreEvent`
when some buffer's diff hunks changed, even if the git status of the
buffer's file didn't change (e.g. a second hunk in a file that was
already modified). After this PR, it also subscribes to the individual
`BufferDiff` entities for buffers that have a git status, so the
`GitStore` is freed from that responsibility. This also fixes some real
cases where the previous strategy was not effective in keeping the
project diff up to date (captured in a test).

Release Notes:

- Fixed some cases where the project diff would fail to update in
response to git events.

crates/project/src/project_tests.rs

@@ -8936,10 +8936,7 @@ async fn test_ignored_dirs_events(cx: &mut gpui::TestAppContext) {
     assert_eq!(
         repository_updates.lock().drain(..).collect::<Vec<_>>(),
         vec![
-            RepositoryEvent::Updated {
-                full_scan: true,
-                new_instance: false,
-            },
+            RepositoryEvent::StatusesChanged { full_scan: true },
             RepositoryEvent::MergeHeadsChanged,
         ],
         "Initial worktree scan should produce a repo update event"
@@ -9000,7 +8997,6 @@ async fn test_ignored_dirs_events(cx: &mut gpui::TestAppContext) {
         repository_updates
             .lock()
             .iter()
-            .filter(|update| !matches!(update, RepositoryEvent::PathsChanged))
             .cloned()
             .collect::<Vec<_>>(),
         Vec::new(),
@@ -9104,17 +9100,10 @@ async fn test_odd_events_for_ignored_dirs(
     });
 
     assert_eq!(
-        repository_updates
-            .lock()
-            .drain(..)
-            .filter(|update| !matches!(update, RepositoryEvent::PathsChanged))
-            .collect::<Vec<_>>(),
+        repository_updates.lock().drain(..).collect::<Vec<_>>(),
         vec![
-            RepositoryEvent::Updated {
-                full_scan: true,
-                new_instance: false,
-            },
             RepositoryEvent::MergeHeadsChanged,
+            RepositoryEvent::BranchChanged
         ],
         "Initial worktree scan should produce a repo update event"
     );
@@ -9142,7 +9131,6 @@ async fn test_odd_events_for_ignored_dirs(
         repository_updates
             .lock()
             .iter()
-            .filter(|update| !matches!(update, RepositoryEvent::PathsChanged))
             .cloned()
             .collect::<Vec<_>>(),
         Vec::new(),