coverage: Ignore lines that can't be covered

[def-]

See the comment line here for example:
https://buildkite.com/materialize/coverage/builds/22#01878e65-e544-4217-a19e-55fe86c699d4

Motivation

• This PR fixes a previously unreported bug.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered.
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• This PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way) and therefore is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• This PR includes the following user-facing behavior changes:

[philip-stoev]

He he well we should expect developers to write enough tests to cover the comment lines too! Not a step back on code coverage!

Thanks for the fix, it made sense after a while -- the second comment line was added by the PR while the first was from before the PR in question.