Use macro NUM_MERGE_MATCH_KINDS instead of '3' in MERGE code.
Code quality improvement for
0294df2f1f84.
Aleksander Alekseev, reviewed by Richard Guo.
Discussion: https://postgr.es/m/CAJ7c6TMsiaV5urU_Pq6zJ2tXPDwk69-NKVh4AMN5XrRiM7N%2BGA%40mail.gmail.com