bundle2: fix broken compression engine assertion

bundletype() is a function, so it needs to be called, and it is
documented to return a 2-tuple. This code is untested, so that's why
we haven't noticed the bad assertion.

Differential Revision: https://phab.mercurial-scm.org/D5155

mercurial/bundle2.py

@@ -839,7 +839,7 b' class unbundle20(unpackermixin):'
             params = self._readexact(paramssize)
             self._processallparams(params)
             yield params
-            assert self._compengine.bundletype == 'UN'
+            assert self._compengine.bundletype()[1] == 'UN'
         # From there, payload might need to be decompressed
         self._fp = self._compengine.decompressorreader(self._fp)
         emptycount = 0