add new methods:

  TROOT::GetSvnRevision()  -- returns svn current revision number
  TROOT::GetSvnBranch()    -- returns svn current branch name
  TROOT::GetSvnDate()      -- returns date time of built
the methods are used in TRint to print in the ROOT welcome message.


git-svn-id: http://root.cern.ch/svn/root/trunk@20151 27541ba8-7e3a-0410-8455-c3a389f83636