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/git/blame.rs

@@ -16,7 +16,7 @@ use markdown::Markdown;
 use multi_buffer::{MultiBuffer, RowInfo};
 use project::{
     Project, ProjectItem as _,
-    git_store::{GitStoreEvent, Repository, RepositoryEvent},
+    git_store::{GitStoreEvent, Repository},
 };
 use smallvec::SmallVec;
 use std::{sync::Arc, time::Duration};
@@ -235,8 +235,8 @@ impl GitBlame {
         let git_store = project.read(cx).git_store().clone();
         let git_store_subscription =
             cx.subscribe(&git_store, move |this, _, event, cx| match event {
-                GitStoreEvent::RepositoryUpdated(_, RepositoryEvent::Updated { .. }, _)
-                | GitStoreEvent::RepositoryAdded(_)
+                GitStoreEvent::RepositoryUpdated(_, _, _)
+                | GitStoreEvent::RepositoryAdded
                 | GitStoreEvent::RepositoryRemoved(_) => {
                     log::debug!("Status of git repositories updated. Regenerating blame data...",);
                     this.generate(cx);