tests/test-mq-qsave.t

formatter: extract function that encode values to json string

This is the stub for tuple support, which will be used to encode ctx.date()
in the same manner as jsonchangeset printer.

  $ 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