Put git-replay-merge in source tree

Let's please have the latest version of git-replay-merge in util/git-replay-merge.sh (or util/git/git-replay-merge.sh, if you think we might be creating other special-use git tools). That way, whenever we check out the BIND repo on a new machine, we'll have the script right there. (As a bonus, it'll be under version control, which is always nice.)