git: Implement a basic repository selector (#23419)

This PR adds a rough-and-ready picker for selecting which of the
project's repositories the git panel should display.

Release Notes:

- N/A

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Nate <nate@zed.dev>

crates/git_ui/src/git_ui.rs

@@ -6,6 +6,7 @@ use ui::{Color, Icon, IconName, IntoElement};
 
 pub mod git_panel;
 mod git_panel_settings;
+pub mod repository_selector;
 
 pub fn init(cx: &mut AppContext) {
     GitPanelSettings::register(cx);