tests/test-mq-qsave.t

copies: clarify the return of _merge_copies_dict

I misused that function twice in the past few days, so lets clarify the API.

Differential Revision: https://phab.mercurial-scm.org/D9418

  $ 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