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/git_ui/src/git_ui.rs

@@ -0,0 +1 @@
+pub mod git_panel;