[rubygems/rubygems] Reduce excess index creation and merging

When @allow_cached is true, @allow_local is always true,
therefore, the #installed_specs will always be merged after #cached_specs
is called. This makes starting with installed_specs.dup redundant.

When #cached_specs is called because @allow_remote is true and
@allow_cached is false, then installed_specs will be added after
cached_specs based on @allow_local.

We never need to add installed_specs here, so don't.

https://github.com/rubygems/rubygems/commit/49b38f9750

lib/bundler/source/rubygems.rb

@@ -380,7 +380,7 @@ module Bundler
 
       def cached_specs
         @cached_specs ||= begin
-          idx = @allow_local ? installed_specs.dup : Index.new
+          idx = Index.new
 
           Dir["#{cache_path}/*.gem"].each do |gemfile|
             s ||= Bundler.rubygems.spec_from_gem(gemfile)