A test pins `mergeForCheck`: the check runs on the merge with today's base, in the pull's own repository, and a conflict is said as the change's fault #6716

src/checks/merged.ts is why a green verdict means the pull passes on today's base. No test calls it directly.

Done when test/merged-check.test.ts builds a base and a pull in $TMPDIR and shows:

• [ ] With the base moved ahead on another file, the merge commit's parents are the base tip and the head, and its tree has both changes.
• [ ] A head already on the base answers without a new commit, or pin what it does.
• [ ] A conflicting head fails with the message that starts with the module's conflict prefix, and no branch or ref moves.
• [ ] The merge commit lives in the pull's repository, and the base repository's refs are unchanged.

Files: test/merged-check.test.ts, src/checks/merged.ts