Summary:
Some of our upcoming repo merges will make it infeasible for someone to
use a full checkout. Let's add a config that will warn users of this. It has a
few levels, starting with a suppressable hint, then a non-suppressable warning,
then a suppressable exception, then a non-suppressable exception.
Reviewed By: ikostia
Differential Revision: D19974408
fbshipit-source-id: bad35a477ad8626dbc0977465368f5d71007e2d5
