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 *is* "rev0"). Differential Revision: https://phab.mercurial-scm.org/D5113