Add Issue related Merge Request fix #1144

Author: LeJeanbono

gitlab4j-api/src/main/java/org/gitlab4j/api/IssuesApi.java

@@ -1326,4 +1326,16 @@ public Issue moveIssue(Object projectIdOrPath, Long issueIid, Object toProjectId
                 "move");
         return (response.readEntity(Issue.class));
     }
+
+    public List<MergeRequest> getIssueRelatedMergeRequests(Object projectIdOrPath, Long issueIid) throws GitLabApiException {
+        Response response = get(
+            Response.Status.OK,
+            null,
+            "projects",
+            getProjectIdOrPath(projectIdOrPath),
+            "issues",
+            issueIid,
+            "related_merge_requests");
+        return (response.readEntity(new GenericType<List<MergeRequest>>() {}));
+    }
 }