WIP: Fix of 'Commits being passed to custom hooks are already reachable when using the UI'

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/19771 This MR changes the way how we create commit, we create it directly in the branch and we don't use temporary ref anymore. This also works similar to how git-receive-pack works so it's a natural way to create commit.

• Replace reference to gitlab_git gem in Gemfile when one released with our change (https://gitlab.com/gitlab-org/gitlab_git/merge_requests/102)
• Specs
• Fix CreateBranchService
• Back-port to CE (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5337)