This patch introduces "oneshot_lines" mode for `Coverage.start`, which
checks "whether each line was executed at least once or not", instead of
"how many times each line was executed". A hook for each line is fired
at most once, and after it is fired, the hook flag was removed; it runs
with zero overhead.
See [Feature #15022] in detail.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65195 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
