Add link to compare merge request versions

It adds a dropdown to the right of merge request version dropdown and
allow you to choose older version for compare. Once clicked it will take
user to compare page with older and newer versions sha pre-filled
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/helpers/merge_requests_helper.rb

@@ -100,4 +100,8 @@ module MergeRequestsHelper
   def merge_request_button_visibility(merge_request, closed)
     return 'hidden' if merge_request.closed? == closed || (merge_request.merged? == closed && !merge_request.closed?) || merge_request.closed_without_fork?
   end
+
+  def compare_versions_path(project, version, base_version)
+    namespace_project_compare_path(project.namespace, project, base_version.head_commit_sha, version.head_commit_sha)
+  end
 end

app/views/projects/merge_requests/show/_versions.html.haml

@@ -22,10 +22,32 @@
                 #{number_with_delimiter(merge_request_diff.commits.count)} #{'commit'.pluralize(merge_request_diff.commits.count)},
                 = time_ago_with_tooltip(merge_request_diff.created_at)
 
+    %span.prepend-left-default
+      Compared with:
+      %span.dropdown.inline
+        %a.btn-link.dropdown-toggle{ data: {toggle: :dropdown} }
+          %strong.monospace base
+          %span.caret
+        %ul.dropdown-menu.dropdown-menu-selectable
+          - merge_request_diffs.each do |merge_request_diff|
+            - next if merge_request_diff.id >= @merge_request_diff.id
+            %li
+              = link_to compare_versions_path(@project, @merge_request_diff, merge_request_diff), class: ('is-active' if merge_request_diff == @base_version), target: '_blank' do
+                %strong.monospace
+                  #{merge_request_diff.head_commit.short_id}
+                %br
+                %small
+                  = time_ago_with_tooltip(merge_request_diff.created_at)
+          %li
+            = link_to '#', class: 'is-active' do
+              %strong.monospace
+                base
+
     - unless @merge_request_diff.latest?
       %span.prepend-left-default
         = icon('info-circle')
         This version is not the latest one. Comments are disabled
+
     .pull-right
       %span.monospace
         #{@merge_request_diff.base_commit.short_id}..#{@merge_request_diff.head_commit.short_id}