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/Cargo.toml

@@ -31,7 +31,7 @@ theme.workspace = true
 ui.workspace = true
 util.workspace = true
 workspace.workspace = true
-worktree.workspace = true
+picker.workspace = true
 
 [target.'cfg(windows)'.dependencies]
 windows.workspace = true