Fix a foreign-repository reverse merge bug that I happened to notice by
inspection.  Foreign repository mergeinfo could have affected the target
mergeinfo in this case.

* subversion/libsvn_client/merge.c
  (prepare_merge_props_changed): Filter out any foreign repository mergeinfo
    unconditionally. This filtering was wrongly being skipped for a reverse
    merge.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1430310 13f79535-47bb-0310-9956-ffa450edef68