On the fsfs-format7 branch: Move the string matching functions
from xdelta to libsvn_subr/string.

* subversion/include/private/svn_string_private.h
  (svn_cstring__match_length,
   svn_cstring__reverse_match_length): declare new private API

* subversion/libsvn_subr/string.c
  (svn_cstring__match_length,
   svn_cstring__reverse_match_length): take implementation from xdelta.c

* subversion/libsvn_delta/xdelta.c
  (match_length,
   reverse_match_length): remove here
  (find_match,
   store_delta_trailer,
   compute_delta): update callers

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1451129 13f79535-47bb-0310-9956-ffa450edef68