xdiff: avoid more compiler warnings with XDL_FAST_HASH on 32-bit machines

xdiff: avoid more compiler warnings with XDL_FAST_HASH on 32-bit machines

Hide literals that can cause compiler warnings for 32-bit architectures in
expressions that evaluate to small numbers there.  Some compilers warn that
0x0001020304050608 won't fit into a 32-bit long, others that shifting right
by 56 bits clears a 32-bit value completely.

The correct values are calculated in the 64-bit case, which is all that matters
in this if-branch.

Reported-by: Øyvind A. Holm <redacted>
Signed-off-by: Rene Scharfe <redacted>
Acked-by: Thomas Rast <redacted>
Signed-off-by: Junio C Hamano <redacted>