shortest: remove unnecessary check for revnum in isvalid()
We now always disambiguating against revnums after we've found an
otherwise valid prefix, so we can safely remove the early isrev()
check in isvalid().
Differential Revision: https://phab.mercurial-scm.org/D3501
$ 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