install newer git, gitlab needs >1.7.10

Install a newer git version on el6.

su - git -c "cd gitlab;bundle exec rake gitlab:check RAILS_ENV=production" says:
Your git bin path is "/usr/bin/git"
Git version >= 1.7.10 ? ... no
  Try fixing it:
  Update your git to a version >= 1.7.10 from Unknown
  Please fix the error above and rerun the checks.