Add git-verify-tag script

Here is a script to simplify validating the gpg signature created by
git-tag-script. Might be useful to add to the git tree so that people
don't have to search for the right post in the git mailinglist archives

git-verify-tag-script

+#!/bin/sh
+GIT_DIR=${GIT_DIR:-.git}
+
+tag=$1
+[ -f "$GIT_DIR/refs/tags/$tag" ] && tag=$(cat "$GIT_DIR/refs/tags/$tag")
+
+git-cat-file tag $tag > .tmp-vtag || exit 1
+cat .tmp-vtag | sed '/-----BEGIN PGP/Q' | gpg --verify .tmp-vtag -
+rm -f .tmp-vtag