py3: make contrib/undumprevlog use absolute_import

contrib/undumprevlog

@@ -3,8 +3,16 b''
 # $ hg init
 # $ undumprevlog < repo.dump
 
+from __future__ import absolute_import
+
 import sys
-from mercurial import revlog, node, scmutil, util, transaction
+from mercurial import (
+    node,
+    revlog,
+    scmutil,
+    transaction,
+    util,
+)
 
 for fp in (sys.stdin, sys.stdout, sys.stderr):
     util.setbinary(fp)