Add a private lookup API for sorted arrays.  This one is easier
to use than svn_sort__bsearch_lower_bound as it will return 
actual matches only, i.e. requires less post-processing for
certain callers.

* subversion/include/svn_sorts.h
  (svn_sort__array_lookup):  Declare the new private API function.

* subversion/libsvn_subr/sorts.c
  (svn_sort__array_lookup):  Implement it.

* subversion/libsvn_fs_fs/index.c
  (get_p2l_entry_from_cached_page,
   svn_fs_fs__p2l_entry_lookup): Simplify using the new API.

* subversion/libsvn_fs_x/index.c
  (get_p2l_entry_from_cached_page,
   p2l_entry_lookup): Ditto.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1554622 13f79535-47bb-0310-9956-ffa450edef68