ci/github-script/get-pr-commit-details: output file list for merge commits

Fixes a bug that was originally found by Claude and reported to the NixOS
security team by Anthropic as a security issue.

Previously, the file list was never output for merge commits.

This could have allowed someone who buried an edit to a file that shouldn't be
edited manually in a merge commit to not trigger the check.

It isn't a security issue, though, because the manual-file-edits check is not
a security boundary (just a friendly reminder to committers).

Anthropic report ID: ANT-2026-223Q3FSE

ci/github-script/get-pr-commit-details.js

@@ -84,6 +84,7 @@ async function getCommitDetailsForPR({ core, pr, repoPath }) {
             'log',
             '--format=%s\t%aN\t%aE\t%cN\t%cE',
             '--name-only',
+            '-m',
             '-1',
             sha,
           ],