changegroup: restore original behavior of _nextclrevtolocalrev

0548f696795b accidentally changed the behavior of cgpacker._close().
The old behavior moved _nextclrevtolocalrev to _clrevtolocalrev only
when _nextclrevtolocalrev was present and then removed
_nextclrevtolocalrev. The bad behavior performed this move
then cleared _clrevtolocalrev because it was the same object as
_nextclrevtolocalrev.

This commit restores the previous behavior.

Surprisingly, no tests changed as a result of this bad logic. I'm
not sure why.

Differential Revision: https://phab.mercurial-scm.org/D4155

mercurial/changegroup.py

@@ -604,9 +604,9 b' class cgpacker(object):'
     def _close(self):
         # Ellipses serving mode.
         self._clrevtolocalrev.clear()
-        if self._nextclrevtolocalrev:
+        if self._nextclrevtolocalrev is not None:
             self._clrevtolocalrev = self._nextclrevtolocalrev
-            self._nextclrevtolocalrev.clear()
+            self._nextclrevtolocalrev = None
         self._changelogdone = True
 
         return closechunk()