shortest: fix an off-by-1 when disambiguating against revnums
Revision "5" is an unambiguous hex prefix in a repo with 5 revisions;
it's only "4" and lower that are ambiguous.
Differential Revision: https://phab.mercurial-scm.org/D4420
#require docutils gettext $ $TESTDIR/check-gendoc el checking for parse errors