tests/test-mq-qsave.t

phabricator: change "readpatch" to be more flexible

Previously, `readpatch` and `querydrev` take a same `params` and `stack`
parameters. This patch changes `readpatch` so it takes the output of
`querydrev`, not the input of `querydrev`. This makes the code more
flexible and cleaner.

Differential Revision: https://phab.mercurial-scm.org/D124

  $ 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