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