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
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