diff: factor out add_diff_options()

Add a function for appending the parseopts member of struct diff_options
to a struct option array.  Use it in two sites instead of accessing the
parseopts member directly.  Decoupling callers from diff internals like
that allows us to change the latter.

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

diff-no-index.c

@@ -255,8 +255,7 @@ int diff_no_index(struct rev_info *revs,
 	};
 	struct option *options;
 
-	options = parse_options_concat(no_index_options,
-				       revs->diffopt.parseopts);
+	options = add_diff_options(no_index_options, &revs->diffopt);
 	argc = parse_options(argc, argv, revs->prefix, options,
 			     diff_no_index_usage, 0);
 	if (argc != 2) {