builtin/fetch.c: don't free remote->name after fetch

Make fetch's string_list of remote names own all of its string items
(strdup'ing when necessary) so that it can deallocate them safely
when clearing.

Signed-off-by: Keith McGuigan <kmcguigan@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>