Document the fact that svnpredumpfilter.py ignores mergeinfo,
unlike svndumpfilter.

* tools/server-side/svnpredumpfilter.py: In the help text, recommend
   svndumpfilter's --skip-missing-merge-sources option since filtering
   with the resulting path list may fail if the option isn't used.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1626186 13f79535-47bb-0310-9956-ffa450edef68