Summary:
Read the Git-LFS specification [1] and implement most checks to prevent
programming error and detect data corruption.
The new code should be stronger than what was before the refactoring, since the
old code only checks keys but not values.
[1]: https://github.com/git-lfs/git-lfs/blob/master/docs/spec.md
Test Plan: Added a new test
Reviewers: #mercurial, davidsp, rmcelroy
Reviewed By: rmcelroy
Subscribers: mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D5046740
Signature: t1:5046740:1494543072:2f0dbfc07cd52a10572550a277d850bdf3f78d27
