git_ui: Feature flag repo selector (#23470)

Fixes an issue where the repo selector showed for all users, not just
those in the git_ui feature flag.

This was meant to be included in the `git_ui` feature flag.

Release Notes:

- N/A

Cargo.lock

@@ -5242,6 +5242,7 @@ dependencies = [
  "collections",
  "db",
  "editor",
+ "feature_flags",
  "futures 0.3.31",
  "git",
  "gpui",
@@ -5253,6 +5254,7 @@ dependencies = [
  "serde_derive",
  "serde_json",
  "settings",
+ "smol",
  "theme",
  "ui",
  "util",