Fix the project diff sometimes missing updates

[cole-miller]

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.