checkout: add missing const to describe_detached_head

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -297,7 +297,7 @@ static void show_local_changes(struct object *head, struct diff_options *opts)
 	run_diff_index(&rev, 0);
 }
 
-static void describe_detached_head(char *msg, struct commit *commit)
+static void describe_detached_head(const char *msg, struct commit *commit)
 {
 	struct strbuf sb = STRBUF_INIT;
 	struct pretty_print_context ctx = {0};