project.rb

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.