untracked-cache: simplify parsing by dropping "next"

untracked-cache: simplify parsing by dropping "next"

When we parse an on-disk untracked cache, we have two pointers, "data"
and "next". As we parse, we point "next" to the end of an element, and
then later update "data" to match.

But we actually don't need two pointers. Each parsing step can just
update "data" directly from other variables we hold (and we don't have
to worry about bailing in an intermediate state, since any parsing
failure causes us to immediately discard "data" and return).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>