Add a prototype with a multi buffer having all project git changes (#21543)

Part of https://github.com/zed-industries/zed/issues/20925

This prototype is behind a feature flag and being merged to avoid
conflicts with further git-related resturctures.
To be a proper, public feature, this needs at least:
* showing deleted files
* better performance 
* randomized tests
* `TODO`s in the `project_diff.rs` file fixed

The good thing is, >90% of the changes are in the `project_diff.rs` file
only, have a basic test and already work on simple cases.

Release Notes:

- N/A

---------

Co-authored-by: Thorsten Ball <thorsten@zed.dev>
Co-authored-by: Cole Miller <cole@zed.dev>

crates/project_panel/src/project_panel.rs

@@ -304,7 +304,7 @@ impl ProjectPanel {
                     cx.notify();
                 }
                 project::Event::WorktreeUpdatedEntries(_, _)
-                | project::Event::WorktreeAdded
+                | project::Event::WorktreeAdded(_)
                 | project::Event::WorktreeOrderChanged => {
                     this.update_visible_entries(None, cx);
                     cx.notify();