Don't apply --destdir twice when running setup.rb

Prior to this patch, if I ran:

    ruby setup.rb --destdir /foo

Then Bundler files would be written into /foo/foo, because destdir was
being prepended, even though `bundler_spec.bin_dir` already included
destdir.

lib/rubygems/commands/setup_command.rb

@@ -385,6 +385,18 @@ def install_default_bundler_gem(bin_dir)
 
     bundler_spec = Gem::Specification.load(default_spec_path)
 
+    # The base_dir value for a specification is inferred by walking up from the
+    # folder where the spec was `loaded_from`. In the case of default gems, we
+    # walk up two levels, because they live at `specifications/default/`, whereas
+    # in the case of regular gems we walk up just one level because they live at
+    # `specifications/`. However, in this case, the gem we are installing is
+    # misdetected as a regular gem, when it's a default gem in reality. This is
+    # because when there's a `:destdir`, the `loaded_from` path has changed and
+    # doesn't match `Gem.default_specifications_dir` which is the criteria to
+    # tag a gem as a default gem. So, in that case, write the correct
+    # `@base_dir` directly.
+    bundler_spec.instance_variable_set(:@base_dir, File.dirname(File.dirname(specs_dir)))
+
     # Remove gemspec that was same version of vendored bundler.
     normal_gemspec = File.join(Gem.default_dir, "specifications", "bundler-#{bundler_spec.version}.gemspec")
     if File.file? normal_gemspec
@@ -399,7 +411,6 @@ def install_default_bundler_gem(bin_dir)
     end
 
     bundler_bin_dir = bundler_spec.bin_dir
-    bundler_bin_dir = File.join(options[:destdir], bundler_bin_dir) unless Gem.win_platform?
     mkdir_p bundler_bin_dir, :mode => 0755
     bundler_spec.executables.each do |e|
       cp File.join("bundler", bundler_spec.bindir, e), File.join(bundler_bin_dir, e)

test/rubygems/test_gem_commands_setup_command.rb

@@ -246,6 +246,28 @@ def test_install_default_bundler_gem_with_force_flag
     end
   end
 
+  def test_install_default_bundler_gem_with_destdir_flag
+    @cmd.extend FileUtils
+
+    bin_dir = File.join(@gemhome, 'bin')
+
+    bindir(bin_dir) do
+      destdir = File.join(@tempdir, 'foo')
+
+      @cmd.options[:destdir] = destdir
+
+      @cmd.install_default_bundler_gem bin_dir
+
+      bundler_spec = Gem::Specification.load("bundler/bundler.gemspec")
+      default_spec_path = File.join(Gem.default_specifications_dir, "#{bundler_spec.full_name}.gemspec")
+      spec = Gem::Specification.load(default_spec_path)
+
+      spec.executables.each do |e|
+        assert_path_exist File.join destdir, spec.bin_dir.gsub(/^[a-zA-Z]:/, ''), e
+      end
+    end
+  end
+
   def test_remove_old_lib_files
     lib                   = File.join @install_dir, 'lib'
     lib_rubygems          = File.join lib, 'rubygems'