CI: Job "lint-submods-branch" unreliable

Example (one of dozens): https://gitlab.haskell.org/ghc/ghc/-/jobs/1207163

Problems with the job:

1. A command within a command substitution fails; the error gets swallowed by bash
2. The command fails because it uses CI_COMMIT_BEFORE_SHA as input to git rev-list, and that var has an invalid value if the pipeline is the first for a MR.

While it would be nice to fix 1, it's probably sufficient to fix 2.

CI_COMMIT_BEFORE_SHA: The previous latest commit present on a branch or tag. Is always 0000000000000000000000000000000000000000 in merge request pipelines and for the first commit in pipelines for branches or tags.
https://docs.gitlab.com/ee/ci/variables/predefined_variables.html