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/editor/src/editor.rs

@@ -156,7 +156,7 @@ use project::{
         },
         session::{Session, SessionEvent},
     },
-    git_store::{GitStoreEvent, RepositoryEvent},
+    git_store::GitStoreEvent,
     lsp_store::{
         CacheInlayHints, CompletionDocumentation, FormatTrigger, LspFormatTarget,
         OpenLspBufferHandle,
@@ -1978,14 +1978,7 @@ impl Editor {
             let git_store = project.read(cx).git_store().clone();
             let project = project.clone();
             project_subscriptions.push(cx.subscribe(&git_store, move |this, _, event, cx| {
-                if let GitStoreEvent::RepositoryUpdated(
-                    _,
-                    RepositoryEvent::Updated {
-                        new_instance: true, ..
-                    },
-                    _,
-                ) = event
-                {
+                if let GitStoreEvent::RepositoryAdded = event {
                     this.load_diff_task = Some(
                         update_uncommitted_diff_for_buffer(
                             cx.entity(),