Add find_insert_index, insert_at_index and clear_func functions to string_list

Add find_insert_index, insert_at_index and clear_func functions to string_list

string_list_find_insert_index() and string_list_insert_at_index()
enables you to see if an item is in the string_list, and to
insert at the appropriate index in the list, if not there.
This is usefull if you need to manipulate an existing item,
if present, and insert a new item if not.

Future mailmap code will use this construct to enable
complex (old_name, old_email) -> (new_name, new_email)
lookups.

The string_list_clear_func() allows to call a custom
cleanup function on each item in a string_list, which is
useful is the util member points to a complex structure.

Signed-off-by: Marius Storm-Olsen <marius@trolltech.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>