rake/backtrace.rb: exclude mere prefix

* lib/rake/backtrace.rb (Rake::Backtrace): exclude mere prefix and
  exec_prefix, which match too much.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47200 b2dd03c8-39d4-4d8f-98ff-823fe69b080e