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/editor/src/editor.rs

@@ -327,6 +327,7 @@ pub fn init(cx: &mut AppContext) {
             .detach();
         }
     });
+    git::project_diff::init(cx);
 }
 
 pub struct SearchWithinRange;

crates/editor/src/git.rs

@@ -1 +1,2 @@
 pub mod blame;
+pub mod project_diff;

crates/editor/src/git/blame.rs

@@ -154,7 +154,7 @@ impl GitBlame {
                         this.generate(cx);
                     }
                 }
-                project::Event::WorktreeUpdatedGitRepositories => {
+                project::Event::WorktreeUpdatedGitRepositories(_) => {
                     log::debug!("Status of git repositories updated. Regenerating blame data...",);
                     this.generate(cx);
                 }