[BreakingChanges] Diff files against themselves to ensure tool doesn't crash

[mikeharder]

The BreakingChanges check has a flaw, where it only runs on a file if there is a diff in the PR. So, if a file has a bug so severe it crashes the BreakingChanges check, it won't be detected until later, when the file is changed. This causes significant confusion, because the changes may be unrelated to the original bug.

To mitigate, check BreakingChanges should run on every file in a PR, even if the file is totally new. Any file can be compared "against itself" just to make sure the tool doesn't crash.