tests/test-mq-qsave.t

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

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "mq=" >> $HGRCPATH

  $ hg init

  $ echo 'base' > base
  $ hg ci -Ambase
  adding base

  $ hg qnew -mmqbase mqbase

  $ hg qsave
  $ hg qrestore 2
  restoring status: hg patches saved state