Make "git checkout" verify that the argument refers to a commit

We still need to create a new branch if it didn't refer to an existing
branch, otherwise our HEAD will continue to point to something totally
different than what we just checked out.

I'll need to think about it.  Maybe only do it with "-f" and force it to
the "master" branch?

git-checkout-script

@@ -13,7 +13,7 @@ while [ "$#" != "0" ]; do
 		force=1
 		;;
 	*)
-		rev=$(git-rev-parse --verify --revs-only "$arg")
+		rev=$(git-rev-parse --verify --revs-only "$arg^0") || exit
 		if [ -z "$rev" ]; then
 			echo "unknown flag $arg"
 			exit 1