Fixes rejected pushes from maintainers
Before the push git would make a call to `/:namespace/:project/git-receive-pack`. This would perform an access check without a ref. So the `Project#branch_allows_maintainer_push?` would return false. This adjusts `Project#branch_allows_maintainer_push?` to return true when passing no branch name if there are merge requests open that would allow the user to push. The actual check then happens when a call to `/api/v4/internal/allowed` is made from a git hook.