Allocate and copy directly in FLEXPTR_ALLOC_MEM and remove the now
unused helper function xalloc_flex(). The resulting code is shorter
and the offset arithmetic is a bit simpler.
Suggested-by: Jeff King <peff@peff.net> Signed-off-by: Rene Scharfe <l.s.r@web.de> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>