New version of fastHadd with correct file permissions.
Style change: use a global variable named commit instead of tag, as we deal with threeish.
Fix comments.
Merge pull request #978 from diguida/fastHadd_file_permissions

fasthadd file permissions
Merge remote branch 'origin/IB/CMSSW_7_1_X/stable' into IB/CMSSW_7_0_X/stable