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/git_ui/src/git_panel.rs

@@ -425,13 +425,20 @@ impl GitPanel {
                     }
                     GitStoreEvent::RepositoryUpdated(
                         _,
-                        RepositoryEvent::Updated { full_scan, .. },
+                        RepositoryEvent::StatusesChanged { full_scan: true }
+                        | RepositoryEvent::BranchChanged
+                        | RepositoryEvent::MergeHeadsChanged,
                         true,
                     ) => {
-                        this.schedule_update(*full_scan, window, cx);
+                        this.schedule_update(true, window, cx);
                     }
-
-                    GitStoreEvent::RepositoryAdded(_) | GitStoreEvent::RepositoryRemoved(_) => {
+                    GitStoreEvent::RepositoryUpdated(
+                        _,
+                        RepositoryEvent::StatusesChanged { full_scan: false },
+                        true,
+                    )
+                    | GitStoreEvent::RepositoryAdded
+                    | GitStoreEvent::RepositoryRemoved(_) => {
                         this.schedule_update(false, window, cx);
                     }
                     GitStoreEvent::IndexWriteError(error) => {