Project Diff 2 (#23891)

This adds a new version of the project diff editor to go alongside the
new git panel.

The basics seem to be working, but still todo:

* [ ] Fix untracked files
* [ ] Fix deleted files
* [ ] Show commit message editor at top
* [x] Handle empty state
* [x] Fix panic where locator sometimes seeks to wrong excerpt

Release Notes:

- N/A

crates/client/src/user.rs

@@ -201,9 +201,8 @@ impl UserStore {
 
                                 cx.update(|cx| {
                                     if let Some(info) = info {
-                                        let disable_staff = std::env::var("ZED_DISABLE_STAFF")
-                                            .map_or(false, |v| !v.is_empty() && v != "0");
-                                        let staff = info.staff && !disable_staff;
+                                        let staff =
+                                            info.staff && !*feature_flags::ZED_DISABLE_STAFF;
                                         cx.update_flags(staff, info.flags);
                                         client.telemetry.set_authenticated_user_info(
                                             Some(info.metrics_id.clone()),