Update merge commit title to mimic github's pattern (#88)

* Update merge commit title to mimic github's pattern

* update based on feedback

Author: chrisdothtml

__tests__/index.js

@@ -61,7 +61,7 @@ Co-authored-by: Test User3 <[email protected]>`;
 
     await probot.receive({name: 'issue_comment', payload: fixtures.payload});
     expect(await mergeRequest).toEqual({
-      commit_title: 'Test PR',
+      commit_title: 'Test PR (#1)',
       commit_message: expectedMessage,
       merge_method: 'squash',
     });

index.js

@@ -69,7 +69,7 @@ module.exports = robot => {
       await github.pullRequests.merge(
         context.repo({
           pull_number: issue.number,
-          commit_title: issue.title,
+          commit_title: `${issue.title} (#${issue.number})`,
           commit_message,
           merge_method,
         }),