check-commit: allow foo_bar naming in functions nameswithallthewordssmashedtogetherarehardtoread. especiallyifenglishisnotyourprimarylanguage. Let's align with the rest of the programming universe and allow_the_use_of_underscores_in_names. We took a hand poll at the 5.2 sprint regarding this change and all but 1 person supported it. The person who didn't expressed concerns around excessive API breakage if we mass renamed things. But we're not planning to mass rename things for the sake of renaming, so all should be well. Differential Revision: https://phab.mercurial-scm.org/D2010