For now (1.8), make svn_fs__canonicalize_abspath always allocate the result
in POOL and re-introduce svn_fs__is_canonical_abspath as a way of checking
that the path content would change.

In the future (1.9), we will hopefully be able to rev the FS API layer and
eliminate the need for canonicalization with the FS implementation.

* subversion/include/private/svn_fs_util.h
  (svn_fs__is_canonical_abspath): re-introduce
  (svn_fs__canonicalize_abspath): update docstring

* subversion/libsvn_fs_util/fs-util.c
  (svn_fs__is_canonical_abspath): implement
  (svn_fs__canonicalize_abspath): always duplicate PATH; optimize string
   termination code

* subversion/libsvn_fs_fs/tree.c
  (open_path,
   get_dag): update 

Suggested by: cmpilato

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1449260 13f79535-47bb-0310-9956-ffa450edef68