Use Index#up_to_date? to skip already-registered plugins

save_plugin grew an equivalent manual path comparison on this branch
while master extracted Index#up_to_date? for the same purpose, so use
the helper instead.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: hsbt

bundler/lib/bundler/plugin.rb

@@ -306,10 +306,9 @@ def validate_plugin!(plugin_path)
     #
     # @raise [PluginInstallError] if validation or registration raises any error
     def save_plugin(name, spec, optional_plugin = false)
-      path = Pathname.new(spec.full_gem_path)
-      return if index.installed?(name) && index.plugin_path(name) == path
+      return if index.up_to_date?(spec)
 
-      validate_plugin!(path)
+      validate_plugin! Pathname.new(spec.full_gem_path)
       installed = register_plugin(name, spec, optional_plugin)
       Bundler.ui.info "Installed plugin #{name}" if installed
     rescue PluginError => e

spec/bundler/plugin_spec.rb

@@ -139,7 +139,7 @@
       end
 
       before do
-        allow(index).to receive(:installed?) { nil }
+        allow(index).to receive(:up_to_date?) { nil }
         allow(definition).to receive(:dependencies) { [Bundler::Dependency.new("new-plugin", ">=0", "plugin" => true), Bundler::Dependency.new("another-plugin", ">=0", "plugin" => true)] }
         allow(installer).to receive(:install_definition) { plugin_specs }
       end