diff-no-index: simplify argv index calculation

Since 16bb3d714d (diff --no-index: use parse_options() instead of
diff_opt_parse(), 2019-03-24) argc must be 2 if we reach the loop, i.e.
argc - 2 == 0.  Remove that inconsequential term.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-no-index.c

@@ -267,7 +267,7 @@ int diff_no_index(struct rev_info *revs,
 	}
 	FREE_AND_NULL(options);
 	for (i = 0; i < 2; i++) {
-		const char *p = argv[argc - 2 + i];
+		const char *p = argv[i];
 		if (!strcmp(p, "-"))
 			/*
 			 * stdin should be spelled as "-"; if you have