Show new branch/merge request button even if a branch / merge request already exists

Description

Continuing discussion from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16854#note_57808040):

After a branch or merge request related to an issue has been created, the "create merge request / branch" button is hidden. However the can be multiple branches / merge requests related to an issue.

Proposal

Always display the button (when user has corresponding permission) to allow creating another branch / merge request.

Links / references

Initially asked here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10018#note_27688662

But I couldn't find the answer.