travis: Avoid timeout on verify-commits check

.travis/lint_06_script.sh

@@ -19,6 +19,7 @@ test/lint/check-rpc-mappings.py .
 test/lint/lint-all.sh
 
 if [ "$TRAVIS_REPO_SLUG" = "bitcoin/bitcoin" -a "$TRAVIS_EVENT_TYPE" = "cron" ]; then
+    git log --merges --before="2 days ago" -1 --format='%H' > ./contrib/verify-commits/trusted-sha512-root-commit
     while read -r LINE; do travis_retry gpg --keyserver hkp://subset.pool.sks-keyservers.net --recv-keys $LINE; done < contrib/verify-commits/trusted-keys &&
-    travis_wait 50 contrib/verify-commits/verify-commits.py;
+    travis_wait 50 contrib/verify-commits/verify-commits.py --clean-merge=2;
 fi