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 merge commits as email patches.

CHANGELOG

@@ -6,6 +6,7 @@ v 7.10.0 (unreleased)
   - enable line wrapping per default and remove the checkbox to toggle it (Hannes Rosenögger)
   - extend the commit calendar to show the actual commits made on a date (Hannes Rosenögger)
   - Add a service to support external wikis (Hannes Rosenögger)
+  - Omit the "email patches" link and fix plain diff view for merge commits
   - List new commits for newly pushed branch in activity view.
   - Add changelog, license and contribution guide links to project sidebar.
   - Improve diff UI

app/views/projects/commit/_commit_box.html.haml

@@ -10,6 +10,7 @@
         Download as
         %span.caret
       %ul.dropdown-menu
+        - unless @commit.parents.length > 1
           %li= link_to "Email Patches", namespace_project_commit_path(@project.namespace, @project, @commit, format: :patch)
         %li= link_to "Plain Diff",    namespace_project_commit_path(@project.namespace, @project, @commit, format: :diff)
     = link_to namespace_project_tree_path(@project.namespace, @project, @commit), class: "btn btn-primary btn-grouped" do