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/semantic_index/src/project_index.rs

@@ -125,7 +125,7 @@ impl ProjectIndex {
         cx: &mut ModelContext<Self>,
     ) {
         match event {
-            project::Event::WorktreeAdded | project::Event::WorktreeRemoved(_) => {
+            project::Event::WorktreeAdded(_) | project::Event::WorktreeRemoved(_) => {
                 self.update_worktree_indices(cx);
             }
             _ => {}