Git improvements (#24238)

- **Base diffs on uncommitted changes**
- **Show added files in project diff view**
- **Fix git panel optimism**

Release Notes:

- Git: update diffs to be relative to HEAD instead of the index; to pave
the way for showing which hunks are staged

---------

Co-authored-by: Cole <cole@zed.dev>

crates/git_ui/src/repository_selector.rs

@@ -49,7 +49,7 @@ impl RepositorySelector {
     fn handle_project_git_event(
         &mut self,
         git_state: &Entity<GitState>,
-        _event: &project::git::Event,
+        _event: &project::git::GitEvent,
         window: &mut Window,
         cx: &mut Context<Self>,
     ) {