builtin-remote.c: plug a small memory leak in get_one_remote_for_updates()

builtin-remote.c: plug a small memory leak in get_one_remote_for_updates()

We know that the string pointed at by remote->name won't change.  It can
be borrowed as the key in the string_list without copying.  Other parts of
existing code such as get_one_entry() already rely on this fact.

Noticed by Cheng Renquan.

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