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/multi_buffer/src/multi_buffer.rs

@@ -166,8 +166,8 @@ impl MultiBufferDiffHunk {
 #[derive(PartialEq, Eq, Ord, PartialOrd, Clone, Hash, Debug)]
 pub struct PathKey {
     // Used by the derived PartialOrd & Ord
-    sort_prefix: Option<u64>,
-    path: Arc<RelPath>,
+    pub sort_prefix: Option<u64>,
+    pub path: Arc<RelPath>,
 }
 
 impl PathKey {
@@ -190,11 +190,6 @@ impl PathKey {
             }
         }
     }
-
-    #[cfg(any(test, feature = "test-support"))]
-    pub fn path(&self) -> &Arc<RelPath> {
-        &self.path
-    }
 }
 
 pub type MultiBufferPoint = Point;