When viewing a file on a sha the edit button should link to the default branch

When I'm on https://gitlab.com/gitlab-org/gitlab-ce/blob/07c32a0df5306082fe800de1ae6e2c51325f46ef/.gitlab-ci.yml the edit button should take me to the edit for the default branch https://gitlab.com/gitlab-org/gitlab-ce/edit/master/.gitlab-ci.yml instead of being greyed out.

Instead I now have to edit the url which is hard to do on mobile.

/cc @jramsay