string-list: align string_list_split() with its _in_place() counterpart

The string_list_split_in_place() function was updated by 52acddf3
(string-list: multi-delimiter `string_list_split_in_place()`,
2023-04-24) to take more than one delimiter characters, hoping that
we can later use it to replace our uses of strtok().  We however did
not make a matching change to the string_list_split() function,
which is very similar.

Before giving both functions more features in future commits, allow
string_list_split() to also take more than one delimiter characters
to make them closer to each other.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

fetch-pack.c

@@ -1914,7 +1914,7 @@ static void fetch_pack_config(void)
 		char *str;
 
 		if (!git_config_get_string("fetch.uriprotocols", &str) && str) {
-			string_list_split(&uri_protocols, str, ',', -1);
+			string_list_split(&uri_protocols, str, ",", -1);
 			free(str);
 		}
 	}