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
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.