revlog: return lazy set from findcommonmissing
When computing the commonmissing, it greedily computes the entire set
immediately. On a large repo where the majority of history is irrelevant, this
causes a significant slow down.
Replacing it with a lazy set makes amend go from 11 seconds to 8.7 seconds.
$ 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