Fix merge request URL passed to Webhooks.

Previously the symbol "url" in the object_attributes hash would always be nil.

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.9.0 (unreleased)
+  - Fix merge request URL passed to Webhooks. (Stan Hu)
   - Move labels/milestones tabs to sidebar
   - Upgrade Rails gem to version 4.1.9.
   - Improve error messages for file edit failures

lib/gitlab/url_builder.rb

@@ -10,6 +10,8 @@ module Gitlab
       case @type
       when :issue
         issue_url(id)
+      when :merge_request
+        merge_request_url(id)
       end
     end
 
@@ -22,5 +24,12 @@ module Gitlab
                                   project_id: issue.project,
                                   host: Gitlab.config.gitlab['url'])
     end
+
+    def merge_request_url(id)
+      merge_request = MergeRequest.find(id)
+      project_merge_request_url(id: merge_request.id,
+                                project_id: merge_request.project,
+                                host: Gitlab.config.gitlab['url'])
+    end
   end
 end

spec/lib/gitlab/url_builder_spec.rb

@@ -8,4 +8,12 @@ describe Gitlab::UrlBuilder do
       expect(url).to eq "#{Settings.gitlab['url']}/#{issue.project.path_with_namespace}/issues/#{issue.iid}"
     end
   end
+
+  describe 'When asking for an merge request' do
+    it 'returns the merge request url' do
+      merge_request = create(:merge_request)
+      url = Gitlab::UrlBuilder.new(:merge_request).build(merge_request.id)
+      expect(url).to eq "#{Settings.gitlab['url']}/#{merge_request.project.to_param}/merge_requests/#{merge_request.id}"
+    end
+  end
 end