cmd_fetch_pack(): respect constness of argv parameter

cmd_fetch_pack(): respect constness of argv parameter

The old code cast away the constness of the strings passed to the
function in argument argv[], which could result in their being
modified by filter_refs().  Fix by copying reference names from argv
and putting them into our own array (similarly to how refnames passed
to stdin were already handled).

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>