This improves the performance in some cases.
`rdoc .../gems/sinatra-2.1.0/README.md` takes 10.5 sec. before this
change, and 7.1 sec. after this change.
`make rdoc` of ruby/ruby takes 19.3 sec. before this change, 18.1 sec.
after this change.
https://github.com/ruby/rdoc/commit/7cf8281e3e