shortest: use 'x' prefix to disambiguate from revnum if configured Differential Revision: https://phab.mercurial-scm.org/D4042

revisions: allow "x123" to refer to nodeid prefix "123" When resolving "123" to a revision, we try to interpret it as revnum before we try to interpret it as a nodeid hex prefix. This can lead to the shortest valid prefix being longer than necessary. This patch lets us write such nodeids in a shorter form by prefixing them with "x" instead of adding more hex digits until they're longer than the longest decimal revnum. On my hg repo with almost 69k revisions, turning this feature on saves on average 0.4% on the average nodeid length. That clearly doesn't justify this patch. However, it becomes more usefule when combined with the earlier patches in this series that let you disambiguate nodeid prefixes within a configured revset. Note that we attempt to resolve symbols as nodeid prefixes after we've exhausted all other posibilities, so this is a backwards compatible change (only queries that would previously fail may now succeed). I've still hidden this feature behind an experiemntal config option so we can roll it back if needed. Differential Revision: https://phab.mercurial-scm.org/D4041