changelog: ensure changelog._delaybuf is initialized

The ``localrepo.writepending`` method is using the ``changelog._delaybuff``
attribute to know if it has anything to do. However the ``changelog._delaybuff``
is never initialised at ``__init__`` time. This can lead to crash when using
bundle2 for part that never touch the changelog.

We simply initialize it to its base value. This is scheduled for stable as it
both trivial and blocking for experimenting with bundle2.

mercurial/changelog.py

@@ -127,6 +127,7 b' class changelog(revlog.revlog):'
             self._generaldelta = False
         self._realopener = opener
         self._delayed = False
+        self._delaybuf = []
         self._divert = False
         self.filteredrevs = frozenset()