fix warning from pack-objects.c

fix warning from pack-objects.c

When compiling on ia64 I get this warning (from gcc 3.4.3):

gcc -o pack-objects.o -c -g -O2 -Wall -DSHA1_HEADER='<openssl/sha.h>'  pack-objects.c
pack-objects.c: In function `pack_revindex_ix':
pack-objects.c:94: warning: cast from pointer to integer of different size

A double cast (first to long, then to int) shuts gcc up, but is there
a better way?

[jc: Andreas Ericsson suggests to use ulong instead. ]

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>