hush: fix shift + $0 bug; add testcase

shell/hush.c

@@ -952,7 +952,7 @@ static int builtin_shift(char **argv)
 		n = atoi(argv[1]);
 	}
 	if (n >= 0 && n < global_argc) {
-		/* XXX This probably breaks $0 */
+		global_argv[n] = global_argv[0];
 		global_argc -= n;
 		global_argv += n;
 		return EXIT_SUCCESS;

shell/hush_test/hush-misc/shift.right

@@ -0,0 +1,2 @@
+./shift.tests abc d e
+./shift.tests d e

shell/hush_test/hush-misc/shift.tests

@@ -0,0 +1,6 @@
+if test $# = 0; then
+    exec "$THIS_SH" $0 abc "d e"
+fi
+echo $0 $1 $2
+shift
+echo $0 $1 $2