svnmucc: Fix an assertion failure.

* subversion/svnmucc/svnmucc.c
  (main): Check for URLs not having a common ancestor and emit an error
    message instead of failing an assertion later.

Patch by: Alexey Neyman <stilor@att.net>
(tweaked by me)

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1511353 13f79535-47bb-0310-9956-ffa450edef68