util: fix human-readable printing of negative byte counts
Apply the same human-readable printing rules to negative byte counts as to
positive ones. Fixes output of debugupgraderepo.
$ 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