Allow multiple 'svnmerge init' commands (with differing copyfrom
parameters, of course) in a single revision.

* contrib/client-side/svnmerge.py
  (check_dir_clean): Modified to error out, only if the change is NOT
  just another modification of the svnmerge-integrated property.
  IOW, dont error out, if the only other change in the working copy
  is an svnmerge-integrated property change.