Fix e2e merge-command assertion broken by the ff-only step (#44)

Since #40 the conflict comment's fast-forward step reads `git merge
--ff-only origin/<branch>`, which `assert_conflict_comment_merges` picks
up with its `^git merge` grep, so the extracted commands never match the
expected conflict merges and the check fails on every conflict scenario.
#40's test tweak only handled the new `# ...` hash gloss, not the new
merge line.

This went unnoticed because `tests.yml` only runs on `pull_request`, so
the breakage only surfaced on the first PR after #40 landed (#42, [run
27235483781](https://github.com/scortexio/autorestack-action/actions/runs/27235483781)).

Fix: skip the `--ff-only` line when extracting merge commands. Verified
green as part of #42's [run
27235912022](https://github.com/scortexio/autorestack-action/actions/runs/27235912022)
before being split out into this PR.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01JHvKryT4QUpHYdNq9YEQxX

---
_Generated by [Claude
Code](https://claude.ai/code/session_01JHvKryT4QUpHYdNq9YEQxX)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: Phlogistique

tests/test_e2e.sh

@@ -291,7 +291,7 @@ assert_conflict_comment_merges() {
         expected+="git merge $conflict"$'\n'
     done
     expected=${expected%$'\n'}
-    actual=$(echo "$comment" | grep -E '^git merge' | sed 's/ *#.*//' || true)
+    actual=$(echo "$comment" | grep -E '^git merge' | grep -v -- '--ff-only' | sed 's/ *#.*//' || true)
 
     if [[ "$actual" == "$expected" ]]; then
         echo >&2 "✅ Verification Passed: conflict comment lists expected merge command(s)."