scmutil: make shortesthexnodeidprefix() use unfiltered repo
Both callers were doing this, and resolvehexnodeidprefix() was also
working on the unfiltered repo, so it makes more sense to have it all
in one place.
Differential Revision: https://phab.mercurial-scm.org/D3313
$ 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