revisions: when using prefixhexnode, ensure we prefix "0"
Previously, if using experimental.revisions.disambiguatewithin (and it didn't
include rev0), and '0' was the shortest identifier in that disambiguation set,
we printed it as the shortest *without* a prefix. This was because we had logic
to determine "if the prefix is a pure integer, but starts with 0, we don't need
to prefix with 'x': 01 is not a synonym for revision #1", but didn't handle the
case where prefix == 0 (which is a pure integer, and starts with 0... but it
Differential Revision: https://phab.mercurial-scm.org/D5113