Expose RefUpdate constructor to any subclass

When we finally move RefDirectory to the new storage.file package,
its associated RefDirectoryUpdate will need visiblity to this
constructor in order to initialize itself.  This is true of any
other repository implementation, so make it protected rather than
package level visible.

Change-Id: If838aec9baeb80ee2f12dcbca717657c725a9242
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/lib/RefUpdate.java

@@ -167,7 +167,16 @@ public abstract class RefUpdate {
 
 	private final Ref ref;
 
-	RefUpdate(final Ref ref) {
+	/**
+	 * Construct a new update operation for the reference.
+	 * <p>
+	 * {@code ref.getObjectId()} will be used to seed {@link #getOldObjectId()},
+	 * which callers can use as part of their own update logic.
+	 *
+	 * @param ref
+	 *            the reference that will be updated by this operation.
+	 */
+	protected RefUpdate(final Ref ref) {
 		this.ref = ref;
 		oldValue = ref.getObjectId();
 		refLogMessage = "";