setdiscovery: don't use dagutil for rev -> node conversions
We don't need to use dagutil to perform a simple rev -> node
conversion.
I haven't measured, but the new code is likely faster, as we
avoid extra function calls and avoid some attribute lookups.
Differential Revision: https://phab.mercurial-scm.org/D4304
$ 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