utils: fix broken assertion in revert_checker

`intermediate_commits` is a list of full SHAs, and `across_ref` may/may
not be a full SHA (or a SHA at all). We already have `across_sha`, which
is the resolved form of `across_ref`, so use that instead.

Thanks to probinson for catching this in post-commit review of
https://reviews.llvm.org/D105578!