replace svninfo.txt by gitinfo.txt to provide git commit and branch information.

Provide git commit and branch info to the system. The methods:
   TROOT::GetSvnRevision()
   TROOT::GetSvnBranch()
   TROOT::GetSvnDate()
have been replaced by:
   TROOT::GetGitCommit()
   TROOT::GetGitBranch()
   TROOT::GetGitDate()
Also the RVersion.h has changed and:
   ROOT_SVN_REVISION
   ROOT_SVN_BRANCH
has been replaced by:
   ROOT_GIT_COMMIT
   ROOT_GIT_BRANCH
Merge branch 'master' of https://root.cern.ch/git/root
forgot to delete svninfo.sh.