Double the speed translate_chunk in case that keyword substitution has
not been enabled.

* subversion/libsvn_subr/subst.c
  (translate_chunk): use a fast scanner if we only care about newlines


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1485848 13f79535-47bb-0310-9956-ffa450edef68