.arcconfig

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

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    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}