We want to use both pre-commit.com and our own `pre-commit.hook`. Since
the hooks are installed only when setting up the dev env, we can assume
there is one and error out in the pre-commit.hook. Also, we can call
pre-commit.com directly from there, as that's what pre-commit.com's
wrapper is doing.
Differential Revision: https://phabricator.freedesktop.org/D1945