* lib/rubygems*: Updated to RubyGems 2.0

* test/rubygems*:  ditto.

* common.mk (prelude):  Updated for RubyGems 2.0 source rearrangement.

* tool/change_maker.rb:  Allow invalid UTF-8 characters in source
  files.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37976 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rubygems/user_interaction.rb

@@ -143,6 +143,16 @@ class Gem::StreamUI
     end
   end
 
+  ##
+  # Prints a formatted backtrace to the errors stream if backtraces are
+  # enabled.
+
+  def backtrace exception
+    return unless Gem.configuration.backtrace
+
+    @errs.puts "\t#{exception.backtrace.join "\n\t"}"
+  end
+
   ##
   # Choose from a list of options.  +question+ is a prompt displayed above
   # the list.  +list+ is a list of option strings.  Returns the pair