bundle2.unpackermixin: default value for seek() whence parameter

The contract for seek() includes seeking to an offset from the beginning of the
file when whence is omitted; put this implementation in compliance.

mercurial/bundle2.py

@@ -497,7 +497,7 b' class unpackermixin(object):'
         """read exactly <size> bytes from the stream"""
         return changegroup.readexactly(self._fp, size)
 
-    def seek(self, offset, whence):
+    def seek(self, offset, whence=0):
         """move the underlying file pointer"""
         if self._seekable:
             return self._fp.seek(offset, whence)