Prevent TFriendElement::GetFile from changin gDirectory/gFile

Properly lookup a chain name in gROOT (instead of the current
directory).  This repairs the case ch->AddFriend(chainname)
when ch is not stored in gROOT.


git-svn-id: http://root.cern.ch/svn/root/trunk@15810 27541ba8-7e3a-0410-8455-c3a389f83636