SubmoduleUpdateCommand#setCallback should return 'this'

The other methods in this class follow the builder pattern, and
return 'this', allowing multiple method calls to be chained in a
single statement.

Update the setCallback method to do the same.
Change-Id: I4ddaacd6d50601f47f61eb6be8b62c8d59cce062
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/api/SubmoduleUpdateCommand.java b/org.eclipse.jgit/src/org/eclipse/jgit/api/SubmoduleUpdateCommand.java
index 34bfbe7..4d3dff0 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/api/SubmoduleUpdateCommand.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/api/SubmoduleUpdateCommand.java
@@ -240,9 +240,11 @@ public SubmoduleUpdateCommand setStrategy(MergeStrategy strategy) {
 	 *
 	 * @param callback
 	 *            the callback
+	 * @return {@code this}
 	 * @since 4.8
 	 */
-	public void setCallback(CloneCommand.Callback callback) {
+	public SubmoduleUpdateCommand setCallback(CloneCommand.Callback callback) {
 		this.callback = callback;
+		return this;
 	}
 }