context: use hex nodeid in error about filtered node

I went a little too far in my cleanup in 9231148ea599 (context:
convert to hex for error message only for 20-byte changeid,
2018-04-06). I missed that the case where a binary nodeid refers to a
filtered node.

Differential Revision: https://phab.mercurial-scm.org/D3987

mercurial/context.py

@@ -411,6 +411,7 b' class changectx(basectx):'
                     self._rev = repo.changelog.rev(changeid)
                     return
                 except error.FilteredLookupError:
+                    changeid = hex(changeid) # for the error message
                     raise
                 except LookupError:
                     # check if it might have come from damaged dirstate