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
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..