reftable: reserve standard PackExt

Reserve "ref" extension for reftable files.  This allows them to be
used in a DFS repository as a stream in a DfsPackDescription.

Change-Id: Ife781bb64d0bb063333183ad2be70a41a2482513

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackExt.java

@@ -59,6 +59,9 @@ public class PackExt {
 	/** A pack bitmap index file extension. */
 	public static final PackExt BITMAP_INDEX = newPackExt("bitmap"); //$NON-NLS-1$
 
+	/** A reftable file. */
+	public static final PackExt REFTABLE = newPackExt("ref"); //$NON-NLS-1$
+
 	/** @return all of the PackExt values. */
 	public static PackExt[] values() {
 		return VALUES;