branchcache: store filtered hash and obsolete hash independently for V3
This will avoid the bug covered in tests/test-branches-obsolete.t when we stop
storing all heads explicitly in V3.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state