remote: clear string_list after use in mv()

remote: clear string_list after use in mv()

Switch to the _DUP variant of string_list for remote_branches to allow
string_list_clear() to release the allocated memory at the end, and
actually call that function.  Free the util pointer as well; it is
allocated in read_remote_branches().

NB: This string_list is empty until read_remote_branches() is called
via for_each_ref(), so there is no need to clean it up when returning
before that point.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>