Add placeholder git panel (#21894)

Adds a simple git placeholder panel for us to iterate from. Also
includes a number of assets from the git prototyping branch that we will
use.

Note: This panel is staff flagged for now.

Release Notes:

- N/A

crates/zed/src/main.rs

@@ -447,6 +447,7 @@ fn main() {
         outline::init(cx);
         project_symbols::init(cx);
         project_panel::init(Assets, cx);
+        git_ui::git_panel::init(cx);
         outline_panel::init(Assets, cx);
         tasks_ui::init(cx);
         snippets_ui::init(cx);