git-checkout-script: use "--verify --revs-only" to parse revs

Sven Verdoolaege points out that I added the --verify option to
git-rev-parse exactly for things like this, but didn't update the
users.

git-checkout-script

@@ -12,7 +12,7 @@ while [ "$#" != "0" ]; do
 		force=1
 		;;
 	*)
-		rev=$(git-rev-parse "$arg")
+		rev=$(git-rev-parse --verify --revs-only "$arg")
 		if [ -z "$rev" ]; then
 			echo "unknown flag $arg"
 			exit 1