py3: use pycompat.ziplist instead of inbuilt zip

zip just like map on Python 3 returns a generator object instead of list. This
results in error if we try to iterate over the result once we consume it. We
have added pycompat.ziplist which returns a list.

Differential Revision: https://phab.mercurial-scm.org/D1790

mercurial/context.py

@@ -1122,7 +1122,7 b' class basefilectx(object):'
                 hist[f] = curr
                 del pcache[f]
 
-        return zip(hist[base][0], hist[base][1].splitlines(True))
+        return pycompat.ziplist(hist[base][0], hist[base][1].splitlines(True))
 
     def ancestors(self, followfirst=False):
         visit = {}