Add related merge requests endpoint

Closes #507

Co-Authored-By: David Windell <[email protected]>

Author: GrahamCampbell

lib/Gitlab/Api/Issues.php

@@ -469,6 +469,17 @@ public function closedByMergeRequests($project_id, $issue_iid)
         return $this->get($this->getProjectPath($project_id, 'issues/'.$this->encodePath($issue_iid)).'/closed_by');
     }
 
+    /**
+     * @param int|string $project_id
+     * @param int        $issue_iid
+     *
+     * @return mixed
+    */
+    public function relatedMergeRequests($project_id, $issue_iid)
+    {
+        return $this->get($this->getProjectPath($project_id, 'issues/'.$this->encodePath($issue_iid) . '/related_merge_requests'));
+    }
+
     /**
      * @param int|string $project_id
      * @param int        $issue_iid

test/Gitlab/Tests/Api/IssuesTest.php

@@ -601,6 +601,26 @@ public function shouldGetIssueClosedByMergeRequests()
         $this->assertEquals($expectedArray, $api->closedByMergeRequests(1, 2));
     }
 
+    /**
+     * @test
+     */
+    public function shouldGetIssueRelatedMergeRequests()
+    {
+        $expectedArray = [
+            ['id' => 1, 'iid' => '1111', 'title' => 'Just saving the world'],
+            ['id' => 2, 'iid' => '1112', 'title' => 'Adding new feature to get merge requests that close an issue'],
+        ];
+
+        $api = $this->getApiMock();
+        $api->expects($this->once())
+            ->method('get')
+            ->with('projects/1/issues/2/related_merge_requests')
+            ->will($this->returnValue($expectedArray))
+        ;
+
+        $this->assertEquals($expectedArray, $api->relatedMergeRequests(1, 2));
+    }
+
     /**
      * @test
      */