Make native svn_fs_move support in FSFS dependent on a format bump
(tweak the conditional manually for testing).  Fall back to ordinary
copy-with-history for older format.

Also, relax the condition on svn_fs_move source revision.  If a rev
older than the txn's base revision is specified, there must have been
no changes at all to that node in meantime and neither the node nor
nor any of the parents been deleted (and later restored).

* subversion/libsvn_fs_fs/fs.h
  (SVN_FS_FS__MIN_MOVE_SUPPORT_FORMAT): declare new feature conditional

* subversion/libsvn_fs_fs/util.h
  (svn_fs_fs__supports_move): declare new private API

* subversion/libsvn_fs_fs/util.c
  (svn_fs_fs__item_offset): implement

* subversion/include/svn_fs.h
  (svn_fs_move): remove condition on from_root, it's now implementation
                 dependent just as other copy-related API are

* subversion/libsvn_fs_fs/tree.c
  (is_changed_node): node comparison helper
  (copy_helper): fall back to copy if move is not supported;
                 allow moves only from txn base rev or equivalent node

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1526057 13f79535-47bb-0310-9956-ffa450edef68