Enable code coverage for commit-validation

It would be beneficial to have code coverage checked as part of commit-validation - specifically the code coverage of the proposed patch, and/or the coverage change the patch introduces.

This could either just be a soft-warning (give CodeReview:-1), or perhaps later a more strict warning (Verified:-1).

Note there's some existing scripts which can do coverage based on a patch extent (possibly clang/llvm's coverage tools - TODO find links for these).