How to control merge behavior in git

Question

Assume that there are two branches A and B in the git library. How to set it to achieve the following effect:
Allow B merge into A, and vice versa

Answer 1

vi .git/hooks/prepare-commit-msg

#!/bin/sh
case "," in
    merge,)
        grep "'A'" ; 
        if [ $? ];then
            echo '禁止合并A merge into B';
            exit 1;
        fi;;
  *) ;;
esac