tests/test-mq-qsave.t

revlogdeltas: split candidate groups selection from the filtering logic

The group iteration has two main components:

* walking candidates, the logic that we are about to extend to build
  intermediate snapshots,

* Making sure we test diffs against interesting bases. No duplicated tests,
  skipping empty revisions, etc.

We split `_candidategroups` to separate the two components. This achieves two
goals:

* It will be simpler to update the walking logic for intermediate snapshots,

* We can gather the filtering logic from `finddeltainfo` into
  `_candidategroups` to centralize it.

  $ 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