revset: remove "small" argument from "_optimize" `_optimize` calculates weights of subtrees. "small" affects some weight calculation (either 1 or 0.5). The weights are now only useful in `and` optimization where we might swap two arguments and use `andsmally`. In the real world, it seems unlikely that revsets with weight of 0.5 or 1 matters the `and` order optimization. I think the important thing is to get weights of expensive revsets right (ex. `contains`). This patch removes the `small` argument to simplify the interface. As for choosing between 0.5 vs 1, things returning a single revision (`ancestor`, `string`) has a weight of 0.5. Things returning multiple revisions returns 1. This could be sometimes useful in the `andsmally` optimization, ex. (((:)-2) & expensive()) & ((1-2) & expensive()) ^^^ ^ ^^^^^^^ ^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^ weight=1 weight=0.5 would have an `andsmally` optimization so `1-2` gets executed first, which seems to be desirable. Differential Revision: https://phab.mercurial-scm.org/D656