Fix a new compiler warning.

* subversion/tests/libsvn_fs_fs/fs-fs-pack-test.c
  (revprop_caching_on_off): Use an intermediate value to avoid a warning
   when passing a pointer-to-non-const-svn_string_t to the
   svn_fs_change_rev_prop2.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1619118 13f79535-47bb-0310-9956-ffa450edef68