transport-helper: plug strbuf and string_list leaks

transport-helper: plug strbuf and string_list leaks

Transfer ownership of detached strbufs to string_lists of the
duplicating variety by calling string_list_append_nodup() instead of
string_list_append() to avoid duplicating and then leaking the buffer.

While at it make sure to release the string_list when done;
push_refs_with_export() already does that.

Reported-by: Jeff King <redacted>
Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>