Fix issue #4370 '(m)erge interactive conflict resolution segfaults when
cwd not parent of target'.

* subversion/svn/file-merge.c
  (svn_cl__merge_file): Don't assume WC_PATH and MERGED_PATH args are
   subtrees of the PATH_PREFIX arg.  This would not be the case if, for
   example, the target of an 'svn up' command was an absolute path which
   is not a child of the current working directory.



git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1488267 13f79535-47bb-0310-9956-ffa450edef68