check-attribution-trailers.sh used `|| true` on `git log`'s exit status,
so a hard failure (bad range, unborn HEAD) and an empty-but-successful
range were indistinguishable — both fell through to the same
`-z "$commits"` exit-0 path. Capture the status via the `if` guard
(exempt from set -e on purpose) and exit 1 on a real git log failure.
check-issue-refs.sh piped `git ls-files | xargs grep | grep -v
lint:allow`, then swallowed the final exit code with `|| true`. Worse:
xargs itself collapses grep's exit 1 (no match) and exit 2+ (real error,
e.g. an unreadable file) into the same xargs(1) status (123 either way),
so even capturing that status can't tell them apart. Switched to `git
grep`, which runs once over the tracked set and hands back its own exit
status untouched (0 matched / 1 no match / 2+ error) — then branch on
that status explicitly for both the scan and the lint:allow filter step.
Refs #4439, #4442
2cb7b5505b