Since we introduced symbolic names for all kinds of node relationships
in r1554800, let's use them instead of magic values everywhere in the
FS layer.  There is no actual change to the logic.

* subversion/libsvn_fs/fs-loader.h
  (id_vtable_t): Use the new enum as the return value of compare.
                 Tweak formatting for consistency.

* subversion/libsvn_fs/fs-loader.c
  (svn_fs_check_related,
   svn_fs_compare_ids): Convert the new FS ID vtable return value type
                        to the FS API level type.

* subversion/libsvn_fs_base/id.h
  (svn_fs_base__id_compare): Update return type to match vtable.

* subversion/libsvn_fs_base/id.c
  (svn_fs_base__id_compare): Update implementation.

* subversion/libsvn_fs_base/tree.c
  (get_copy_inheritance,
   base_node_relation,
   txn_body_copy): Update BDB-internal users of that function. 

* subversion/libsvn_fs_fs/id.h
  (svn_fs_fs__id_compare): Similar change as in BDB.

* subversion/libsvn_fs_fs/id.c
  (svn_fs_fs__id_compare): Ditto.

* subversion/libsvn_fs_fs/tree.c
  (get_copy_inheritance): Update FSFS-internal users of that function.

* subversion/libsvn_fs_x/id.h
  (svn_fs_x__id_compare): Similar change as in BDB.

* subversion/libsvn_fs_x/id.c
  (svn_fs_x__id_compare): Ditto.

* subversion/libsvn_fs_x/tree.c
  (get_copy_inheritance): Update FSX-internal users of that function.

Suggested by: julianfoad

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1574144 13f79535-47bb-0310-9956-ffa450edef68