revlog: fix a regression with null revision

Introduced in the patch which fixes issue3497
Part of that patch was erroneously submitted and it shouldn't be in the code

mercurial/revlog.py

@@ -771,10 +771,6 b' class revlog(object):'
                 nl = [n for n in nl if hex(n).startswith(id)]
                 if len(nl) > 0:
                     if len(nl) == 1:
-                        if nl[0] == nullid:
-                            # dummy null revision always exists,
-                            # it shouldn't be returned here
-                            return None
                         self._pcache[id] = nl[0]
                         return nl[0]
                     raise LookupError(id, self.indexfile,

tests/test-glog.t

@@ -1542,7 +1542,6 b' have 2 filelog topological heads in a li'
   $ testlog --follow
   []
   []
-  abort: unknown revision '0'!
   $ echo a > a
   $ echo aa > aa
   $ echo f > f