merges r21570 from trunk into ruby_1_9_1.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_1@21575 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
merges r21570 from trunk into ruby_1_9_1.
set_trace_func should be completely final task in debug.rb.
Bug #847 [ruby-core:20449]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_1@21575 b2dd03c8-39d4-4d8f-98ff-823fe69b080e