branchcache: gather new obsolete revision in a set
This is part of a series to more clearly split the update in two step. This
will allow us to introduce a fast path during update in a future changeset.
$ 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