planner: don't choose merge join unless there's hint or join key fully matched | tidb-test=pr/2493#59933

ti-chi-bot[bot] merged 7 commits intopingcap:masterpingcap/tidb:masterfrom

winoros:disable-merge-join-unless-fullcoverwinoros/tidb:disable-merge-join-unless-fullcoverCopy head branch name to clipboard