updated fork feature to use gitlab-shell for v5 of gitlab
Fix bug for repeated fork requests

When asking to fork a project and a project with the same name
already exists (likely from a previous fork), the recovery from
the fork failure would inadvertantly delete the repo of the
existing destination project.
Made fixes suggested by @randx in pull request

See https://github.com/gitlabhq/gitlabhq/pull/3597#discussion-diff-4014638
Merge pull request #3597 from amacarthur/fork-pull-request

updated fork feature to use gitlab-shell for v5 of gitlab