bundle2.unbundle20: add compressed() method

Bundlerepo uses the compressed() method to determine whether it should write
an uncompressed temporary file.  Since we don't support compressed bundle2 files
at the moment, make this method return true.

mercurial/bundle2.py

@@ -607,6 +607,8 b' class unbundle20(unpackermixin):'
             return self._readexact(headersize)
         return None
 
+    def compressed(self):
+        return False
 
 class bundlepart(object):
     """A bundle2 part contains application level payload