Remove the email patches link for merge commits
Rugged's `to_mbox` method doesn't work for merge commits, so don't display the option to download them as email patches. This is part of the fix for #1878, and gitlab/gitlab_git!27 allows a plain diff to be generated for a merge commit. Together these changes should prevent the 500 errors described in the issue.
cc @sytse
See merge request !1737
