scmutil: fix shortesthexnodeidprefix on Python 3 for 0-prefixed nodes
This fixes test-bookmarks.t on Python 3 (which had regressed.)
Differential Revision: https://phab.mercurial-scm.org/D3926
$ 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