#1072: read commit_id from underlying JSON in Event.Smart.commitId()

Replaces the stub with Optional.fromNullable(event.json().getString(
"commit_id", null)). When the GitHub API response includes a non-null
commit_id, the method now returns Optional.of(sha); otherwise (null
value or missing key) it returns Optional.absent(), matching the
documented Issue Events payload at developer.github.com/v3/issues/events.

Author: bibonix

src/main/java/com/jcabi/github/Event.java

@@ -233,7 +233,9 @@ public Date createdAt() throws IOException {
          * @since 1.7.0
          */
         public Optional<String> commitId() throws IOException {
-            return Optional.absent();
+            return Optional.fromNullable(
+                this.event.json().getString("commit_id", null)
+            );
         }
 
         /**