string.c: SHARABLE_SUBSTRING_P

* string.c (SHARABLE_SUBSTRING_P): predicate if substring can be
  shared with the original string.  true if just at the end of the
  original string, for the time being.  all substring will be able to
  be shared in the future.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45633 b2dd03c8-39d4-4d8f-98ff-823fe69b080e