revset: optimize missing ancestor expressions A missing ancestor expression is any expression of the form (::x - ::y) or equivalent. Such expressions are remarkably common, and so far have involved multiple walks down the DAG, followed by a set difference operation. With this patch, such expressions will be transformed into uses of the fast algorithm at ancestor.missingancestor. For a repository with over 600,000 revisions, perfrevset for '::tip - ::-10000' returns: Before: ! wall 3.999575 comb 4.000000 user 3.910000 sys 0.090000 (best of 3) After: ! wall 0.132423 comb 0.130000 user 0.130000 sys 0.000000 (best of 75)