parse-options: const parse_options_concat() parameters

Document the fact that the function doesn't modify the two option arrays
passed to it by adding the keyword const to each parameter.

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

parse-options-cb.c

@@ -181,7 +181,8 @@ struct option *parse_options_dup(const struct option *o)
 	return opts;
 }
 
-struct option *parse_options_concat(struct option *a, struct option *b)
+struct option *parse_options_concat(const struct option *a,
+				    const struct option *b)
 {
 	struct option *ret;
 	size_t a_len = parse_options_count(a);

parse-options.h

@@ -281,7 +281,7 @@ int parse_options_step(struct parse_opt_ctx_t *ctx,
 int parse_options_end(struct parse_opt_ctx_t *ctx);
 
 struct option *parse_options_dup(const struct option *a);
-struct option *parse_options_concat(struct option *a, struct option *b);
+struct option *parse_options_concat(const struct option *a, const struct option *b);
 
 /*----- some often used options -----*/
 int parse_opt_abbrev_cb(const struct option *, const char *, int);