pack-objects: convert recursion to iteration in break_delta_chain()

The break_delta_chain() function is recursive over the depth
of a given delta chain, which can lead to possibly running
out of stack space. Normally delta depth is quite small, but
if there _is_ a pathological case, this is where we would
find and fix it, so we should be more careful.

We can do it without recursion at all, but there's a little
bit of cleverness needed to do so. It's easiest to explain
by covering the less-clever strategies first.

The obvious thing to try is just keeping our own stack on
the heap. Whenever we would recurse, push the new entry onto
the stack and loop instead. But this gets tricky; when we
see an ACTIVE entry, we need to care if we just pushed it
(in which case it's a cycle) or if we just popped it (in
which case we dealt with its bases, and no we need to clear
the ACTIVE flag and compute its depth).

You can hack around that in various ways, like keeping a
"just pushed" flag, but the logic gets muddled. However, we
can observe that we do all of our pushes first, and then all
of our pops afterwards. In other words, we can do this in
two passes. First dig down to the base, stopping when we see
a cycle, and pushing each item onto our stack.  Then pop the
stack elements, clearing the ACTIVE flag and computing the
depth for each.

This works, and is reasonably elegant. However, why do we
need the stack for the second pass? We can just walk the
delta pointers again. There's one complication. Popping the
stack went over our list in reverse, so we could compute the
depth of each entry by incrementing the depth of its base,
which we will have just computed.  To go forward in the
second pass, we have to compute the total depth on the way
down, and then assign it as we go.

This patch implements this final strategy, because it not
only keeps the memory off the stack, but it eliminates it
entirely. Credit for the cleverness in that approach goes to
Michael Haggerty; bugs are mine.

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

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