simplify the logic of git-cms-addpkg, plus varous minor bug fixes

simplify the logic of git-cms-addpkg, plus varous minor bug fixes

git-cms-init:
 - fix help message
 - fix for when CMSSW_GIT_REFERENCE is not set
 - properly silence/unsilence messages from git clone and git fetch

git-cms-addpkg:
 - use git cms-init to setup the local repository
 - do not try to change TAG when adding a package

git-cms-sparse-checkout
 - remove the temporary file after the checkout

git-cms-merge-topic
 - use git cms-init to setup the local repository