revset: comment that we can't swap 'or' operands by weight Though the original code did nothing, it tried to optimize the calculation order by weight. But we can't simply swap 'ta' and 'tb' because it would change the order of revisions. For future reference, this patch keeps the modified version of the original code as comment.

parser: add helper to reduce nesting of chained infix operations This will be used to avoid stack overflow caused by chained 'or' operations in revset.

convert: properly pass null ids through .hgtags (issue4678) Mercurial uses tags of null to mark deletions, but convert was silently discarding these because it had no mapping for them. Thus, it was resurrecting deleted tags.