check-commit: allow foo_bar naming in functions
Let's align with the rest of the programming universe and
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