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/git/src/diff.rs

@@ -80,7 +80,6 @@ impl BufferDiff {
         self.tree.is_empty()
     }
 
-    #[cfg(any(test, feature = "test-support"))]
     pub fn hunks_in_row_range<'a>(
         &'a self,
         range: Range<u32>,