Fix fast-forward merge requests git hooks validations

Merged Gabriel Mazetto requested to merge bug/git_hooks into master

Current implementations checks git hooks during merge request action issued by Web UI to prevent a git merge action failing and not reporting back to the UI (introduced by !326 (merged)). The problem is that we shouldn't make the check the merge commit when we are using a Fast Forward merge type.

Fixes gitlab-org/gitlab-ee#598