- In case of non-tty, do not print all progress messages during the merging phase
     to avoid filling up the logs with useless info.
   - Add member TProof::fTty and getter TProof::IsTty() to centrally control the tty-ness
     of the session


git-svn-id: http://root.cern.ch/svn/root/trunk@47444 27541ba8-7e3a-0410-8455-c3a389f83636