builtin/remote: fix leaking strings in `branch_list`

builtin/remote: fix leaking strings in `branch_list`

The `struct string_list branch_list` is declared as `NODUP`, which makes
it not copy strings inserted into it. This causes memory leaks though,
as this means it also won't be responsible for _freeing_ inserted
strings. Thus, every branch we add to this will leak.

Fix this by marking the list as `DUP` instead and free the local copy we
have of the variable.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>