Allow `git config hooks.warn true` to set WARN level

Setting hooks.warn in ~/.gitconfig or $PROJECT/.git/config will be
verbose about the errors that were found, but allow the commit to go
through anyway.

It's strongly recommended to not set this config variable and instead
commit only good code. SKIPLINT=1 can be used in an "emergency" if you
need to commit code that's not passing the git hooks.