scanPacks to return reftables even if no packs

An empty repository may have a dangling symref HEAD pointing to
refs/heads/master.  In this case, there will be a reftable even though
there are no packs yet.

Change-Id: Ib759ffbbfc490953481853e74263dd46d2592888
Signed-off-by: Minh Thai <mthai@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsObjDatabase.java

@@ -546,7 +546,7 @@ public abstract class DfsObjDatabase extends ObjectDatabase {
 			}
 		}
 
-		if (newPacks.isEmpty())
+		if (newPacks.isEmpty() && newReftables.isEmpty())
 			return new PackListImpl(NO_PACKS.packs, NO_PACKS.reftables);
 		if (!foundNew) {
 			old.clearDirty();