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have been replaced by: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()
but for backward compatibility we've retained GetSvnRevision() which now
returns the decimal value of the short SHA1 of GetGitCommit(). So don't
rely on revisions increasing monotonically.

Also the RVersion.h has changed and:
   ROOT_SVN_REVISION
   ROOT_SVN_BRANCH
has been replaced by:
   ROOT_GIT_COMMIT
   ROOT_GIT_BRANCH