reftable: add maxUpdateIndex to Reftable

This makes maxUpdateIndex() available in MergedReftable, so we can
know generically at which index to create the next reftable in a
stack.

Change-Id: Ia2314bc57c8b5dd7e69d5e61096fdce1d35abd11
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/MergedReftable.java

@@ -91,6 +91,15 @@ public class MergedReftable extends Reftable {
 		}
 	}
 
+	/**
+	 * {@inheritDoc}
+	 */
+	@Override
+	public long maxUpdateIndex() throws IOException {
+		return tables.length > 0 ? tables[tables.length - 1].maxUpdateIndex()
+				: 0;
+	}
+
 	/** {@inheritDoc} */
 	@Override
 	public RefCursor allRefs() throws IOException {

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/Reftable.java

@@ -98,6 +98,19 @@ public abstract class Reftable implements AutoCloseable {
 		includeDeletes = deletes;
 	}
 
+
+	/**
+	 * Get the maximum update index for log entries that appear in this
+	 * reftable.
+	 *
+	 * @return the maximum update index for log entries that appear in this
+	 *         reftable. This should be 1 higher than the prior reftable's
+	 *         {@code maxUpdateIndex} if this table is used in a stack.
+	 * @throws java.io.IOException
+	 *             file cannot be read.
+	 */
+	public abstract long maxUpdateIndex() throws IOException;
+
 	/**
 	 * Seek to the first reference, to iterate in order.
 	 *

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/reftable/ReftableReader.java

@@ -146,15 +146,9 @@ public class ReftableReader extends Reftable {
 	}
 
 	/**
-	 * Get the maximum update index for log entries that appear in this
-	 * reftable.
-	 *
-	 * @return the maximum update index for log entries that appear in this
-	 *         reftable. This should be 1 higher than the prior reftable's
-	 *         {@code maxUpdateIndex} if this table is used in a stack.
-	 * @throws java.io.IOException
-	 *             file cannot be read.
+	 * {@inheritDoc}
 	 */
+	@Override
 	public long maxUpdateIndex() throws IOException {
 		if (blockSize == -1) {
 			readFileHeader();