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