Shell out fewer times

This is a follow up to #9053. We can avoid shelling out for generating
the gem index.

Author: tenderlove

bundler/spec/spec_helper.rb

@@ -27,6 +27,7 @@
 require "rspec/expectations"
 require "rspec/mocks"
 require "rspec/support/differ"
+require "rubygems/indexer"
 
 require_relative "support/builders"
 require_relative "support/checksums"

bundler/spec/support/builders.rb

@@ -279,14 +279,8 @@ def update_repo(path, build_compact_index: true)
       @_build_path = "#{path}/gems"
       @_build_repo = File.basename(path)
       yield
-      with_gem_path_as scoped_base_system_gem_path do
-        Dir[scoped_base_system_gem_path.join("gems/rubygems-generate_index*/lib")].first ||
-          raise("Could not find rubygems-generate_index lib directory in #{scoped_base_system_gem_path}")
-
-        command = "generate_index"
-        command += " --no-compact" if !build_compact_index && gem_command(command + " --help").include?("--[no-]compact")
-        gem_command command, dir: path
-      end
+      options = { build_compact: build_compact_index }
+      Gem::Indexer.new(path, options).generate_index
     ensure
       @_build_path = nil
       @_build_repo = nil