ci: notify user when branch is too far off master to be analyzed

We just hit this in https://git.card10.badge.events.ccc.de/swym/firmware/-/jobs/3010.

This currently leaves an annoyingly difficult to parse error:

$ git diff --name-only $(git merge-base --fork-point card10/master)...HEAD | xargs tools/code-style.sh
usage: tools/code-style.sh <source.c> ...