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>

Cargo.lock

@@ -3790,6 +3790,7 @@ dependencies = [
  "db",
  "emojis",
  "env_logger 0.11.5",
+ "feature_flags",
  "file_icons",
  "fs",
  "futures 0.3.31",
@@ -3823,6 +3824,7 @@ dependencies = [
  "snippet",
  "sum_tree",
  "task",
+ "tempfile",
  "text",
  "theme",
  "time",