tests/test-mq-qsave.t

parsers: introduce headrevsfiltered in C extension

All extensions that have this function do support filtering. The existing
headrevs function may support filtering but we cannot reliably detect whether
it does.

  $ 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