tests/test-mq-qsave.t

sparse: refactor sparsechecksum()

This was relying on garbage collection to close the opened
file, which is a bug. Both callers simply called into self.vfs
to resolve the path. So refactor to use the vfs layer.

While we're here, rename the method to reflect it is internal
and to break anyone relying on the old behavior.

  $ 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