This works by defining two 'static' lists in hack. The first is the list
of all flags in the project which use a `-` or an `_` in their name. All
files being processed by verify-flags-underscore.py (or all files in the
repo if no filename arguments are given) will be searched for flag
declaration using a simple regex. Its not super smart. If a flag is
found which is not in the static list it will complain/reject the commit
until a human adds it to the list. If we do not keep a static list of
flags it takes >.2 seconds to find them 'all' at runtime. Since this is
run in pre-commit saving every part of a second helps.

After it finds all of the flags it runs all of the arguments (or all
files in repo if no arguments) looking for usage of those flags which
includes an `_`. There are lots of places where these are false
positives. For example we have a flag named oom-adj-score but the kernel
calls it oom_adj_score. To handle this we keep a second 'whitelist' of
lines which are allowed to use these flag names with an `_`.

Running the entire git repo looking for flags in every golang file and
looking in every single file for bad usage takes about 8.75 seconds.

Running it in the precommit hook where we only check things that changed
takes about .06 seconds.