Fix unexpected fetch_merge plus out status interactions

assets/lib/commands/out.rb

@@ -41,6 +41,13 @@ def output
         version = { 'ref' => sha }
       else
         pr = PullRequest.from_github(repo: repo, id: id)
+        if sha == pr.merge_commit_sha
+          # If the SHA that we have here is the merge commit's SHA,
+          # we know that the correct SHA is the one at the tip of
+          # the PR (which might be the same as the merge SHA if
+          # there's no need for a merge).
+          sha = pr.sha
+        end
         metadata << { 'name' => 'url', 'value' => pr.url }
         version = { 'pr' => id, 'ref' => sha }
       end
@@ -77,7 +84,7 @@ def output
                        File.read(commit_path, encoding: Encoding::UTF_8)
                      else
                        ''
-        end
+                     end
         Octokit.merge_pull_request(input.source.repo, id, commit_msg, merge_method: params.merge.method, accept: 'application/vnd.github.polaris-preview+json')
         metadata << { 'name' => 'merge', 'value' => params.merge.method }
         metadata << { 'name' => 'merge_commit_msg', 'value' => commit_msg }

assets/lib/pull_request.rb

@@ -47,6 +47,10 @@ def sha
     @pr['head']['sha']
   end
 
+  def merge_commit_sha
+    @pr['merge_commit_sha']
+  end
+
   def url
     @pr['html_url']
   end

spec/commands/out_spec.rb

@@ -303,6 +303,23 @@ def stub_json(method, uri, body)
                                  { 'name' => 'url', 'value' => 'http://example.com' }
                                ])
         end
+        context 'when fetching a merged commit' do
+          it 'posts the status to the PR\'s SHA instead of the merge SHA.' do
+            stub_json(:get, 'https://api.github.com:443/repos/jtarchie/test/pulls/1',
+                      html_url: 'http://example.com',
+                      number: 1,
+                      head: { sha: 'abcdef' },
+                      merge_commit_sha: @sha)
+            stub_request(:post, 'https://api.github.com:443/repos/jtarchie/test/statuses/abcdef')
+            output, = put('params' => { 'status' => 'failure', 'path' => 'resource' }, 'source' => { 'repo' => 'jtarchie/test' })
+            expect(output).to eq('version'  => { 'ref' => 'abcdef', 'pr' => '1' },
+                                 'metadata' => [
+                                   { 'name' => 'status', 'value' => 'failure' },
+                                   { 'name' => 'url', 'value' => 'http://example.com' }
+                                 ])
+
+          end
+        end
       end
     end
   end