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

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>