Follow-up to r1442088:  revise the use of open_path() and make
it require canonical values for PATH.  Eliminate the need for
svn_fs__is_canonical_abspath and consequently drop that function.
It is now an internal optimization in svn_fs__canonicalize_abspath.

All callers of svn_fs__canonicalize_abspath have been checked
whether simply returning PATH is o.k.

* subversion/include/private/svn_fs_util.h
  (svn_fs__is_canonical_abspath): drop
  (svn_fs__canonicalize_abspath): state we may simply return PATH.

* subversion/libsvn_fs_util/fs-util.c
  (svn_fs__is_canonical_abspath): rename and simplify to ...
  (is_canonical_abspath): ... this new version
  (svn_fs__canonicalize_abspath): before doing expensive operations,
   check whether canonicalization is necessary

* subversion/libsvn_fs_fs/tree.c
  (open_path_is_canonical): drop
  (open_path_node_only): renumber
  (open_path): assert() PATH parameter to be canonical
  (get_dag,
   fs_change_node_prop,
   fs_make_dir,
   fs_delete_node,
   fs_make_file,
   fs_apply_textdelta,
   fs_apply_text,
   fs_closest_copy, 
   assemble_history,
   get_mergeinfo_for_path_internal): update direct and indirect
   callers ensuring PATH is canonical

Suggested by: julianfoad

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1448810 13f79535-47bb-0310-9956-ffa450edef68