Catches the failure mode that produced #25045: a contributor PR whose
branch had been disconnected from main's history (likely an accidental
'git checkout --orphan' or '.git/' re-init). GitHub's merge UI does
not refuse merges of unrelated histories, so the PR landed cleanly
with its intended one-file change but its parent-less root commit
(413990c94) got grafted into main as a second root. The merge
resolution itself was correct — main's content won for every
conflicting file — but ~1500 files' worth of git blame collapsed
onto that single commit.
Implementation: 'git merge-base origin/main HEAD' exits non-zero and
prints nothing when the two commits share no ancestor. Check both
conditions and fail with a clear message + recovery steps.
Verified: against the historic state of PR #25045 (base 5d90386ba,
head 1149e75db), 'git merge-base' returns empty with exit 1, so the
new check would have rejected it.
