builtin/repack.c: pass "packtmp" to `generated_pack_populate()`

builtin/repack.c: pass "packtmp" to `generated_pack_populate()`

In a similar spirit as previous commits, this function needs to know the
temporary pack prefix, which it currently accesses through the static
"packtmp" variable within builtin/repack.c.

Pass it explicitly as a function parameter to facilitate moving this
function out of builtin/repack.c entirely.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>