revset: add expectsize to check the size of a set `expectsize(<set>, <int>)` revset fails if `<set>` is not exactly `<int>` elements. `expectsize(<set>, <min>:<max>)` revset fails if `<set>` is not exactly between `<min>` and `<max>` inclusive. This then allows an alias for `hg next` to be `update -r one(children(.))` with sane failure behavior, and also makes some other scripting tasks a little less difficult. (Summary from WeShouldDoThat) Differential Revision: https://phab.mercurial-scm.org/D5813