Follow-up to r1618138:  Correct UUID / ID handling during FS upgrade.

* subversion/libsvn_fs_fs/fs_fs.c
  (read_uuid): Factored out from ...
  (svn_fs_fs__open): ... this.
  (upgrade_body): Actually read the UUID before trying to rewrite it.
                  Always update the instance ID upon upgrade.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618151 13f79535-47bb-0310-9956-ffa450edef68