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.c

@@ -5397,6 +5397,12 @@ static int diff_opt_rotate_to(const struct option *opt, const char *arg, int uns
 	return 0;
 }
 
+struct option *add_diff_options(const struct option *opts,
+				struct diff_options *options)
+{
+	return parse_options_concat(opts, options->parseopts);
+}
+
 static void prep_parse_options(struct diff_options *options)
 {
 	struct option parseopts[] = {