If an object/class can not be merger, warn only if there is really a need to...

If an object/class can not be merger, warn only if there is really a need to merge (i.e. the collection of files contains more than one object of this type and name


git-svn-id: http://root.cern.ch/svn/root/trunk@41447 27541ba8-7e3a-0410-8455-c3a389f83636