Fix typo in git-checkout-script.

The confusion between "revs" vs "refs" caused us to not find the branch
name, which in turn meant that we never switched the HEAD over to it.

git-checkout-script

@@ -22,7 +22,7 @@ while [ "$#" != "0" ]; do
 			exit 1
 		fi
 		new="$rev"
-		if [ -f "$GIT_DIR/revs/heads/$arg" ]; then
+		if [ -f "$GIT_DIR/refs/heads/$arg" ]; then
 			branch="$arg"
 		fi
 		;;