addset: do lazy sorting
The previous implementation was consuming the whole revset when asked for any
sort. The addset class is now doing lazy sorting like all other smarset classes.
This has no significant impact in the benchmark as-is. But this is important
to later change.
$ 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