RefDatabase: Update Javadoc for ALL constant

The Javadoc refers to the deprecated getRefs method. Update it to refer
to getRefsByPrefix which is the recommended replacement of getRefs.

Change-Id: I61f2abcf1a3794f40a1746317dbc18aa0beb87a7
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

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

@@ -91,7 +91,10 @@ public abstract class RefDatabase {
 	 */
 	public static final int MAX_SYMBOLIC_REF_DEPTH = 5;
 
-	/** Magic value for {@link #getRefs(String)} to return all references. */
+	/**
+	 * Magic value for {@link #getRefsByPrefix(String)} to return all
+	 * references.
+	 */
 	public static final String ALL = "";//$NON-NLS-1$
 
 	/**