Backup the array passed to fetch_pack so we can free items

Backup the array passed to fetch_pack so we can free items

fetch_pack() can call remove_duplicates() on its input array and
this will possibly overwrite an earlier entry with a later one if
there are any duplicates in the input array.  In such a case the
caller here might then attempt to free an item multiple times as
it goes through its cleanup.

I also forgot to free the heads array we pass down into fetch_pack()
when I introduced the allocation of it in this function during my
builtin-fetch cleanup series.  Better free it while we are here
working on related memory management fixes.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>