scmutil: fix shortesthexnodeidprefix on Python 3 for 0-prefixed nodes

Authored by durin42.

Description

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

Details