attr: avoid recursion when expanding attribute macros

Given a set of attribute macros like:

   [attr]a1 a2
   [attr]a2 a3
   ...
   [attr]a300000 -text
   file a1

expanding the attributes for "file" requires expanding "a1" to "a2",
"a2" to "a3", and so on until hitting a non-macro expansion ("-text", in
this case). We implement this via recursion: fill_one() calls
macroexpand_one(), which then recurses back to fill_one(). As a result,
very deep macro chains like the one above can run out of stack space and
cause us to segfault.

The required stack space is fairly small; I needed on the order of
200,000 entries to get a segfault on Linux. So it's unlikely anybody
would hit this accidentally, leaving only malicious inputs. There you
can easily construct a repo which will segfault on clone (we look at
attributes during the checkout step, but you'd see the same trying to do
other operations, like diff in a bare repo). It's mostly harmless, since
anybody constructing such a repo is only preventing victims from cloning
their evil garbage, but it could be a nuisance for hosting sites.

One option to prevent this is to limit the depth of recursion we'll
allow. This is conceptually easy to implement, but it raises other
questions: what should the limit be, and do we need a configuration knob
for it?

The recursion here is simple enough that we can avoid those questions by
just converting it to iteration instead. Rather than iterate over the
states of a match_attr in fill_one(), we'll put them all in a queue, and
the expansion of each can add to the queue rather than recursing. Note
that this is a LIFO queue in order to keep the same depth-first order we
did with the recursive implementation. I've avoided using the word
"stack" in the code because the term is already heavily used to refer to
the stack of .gitattribute files that matches the tree structure of the
repository.

The test uses a limited stack size so we can trigger the problem with a
much smaller input than the one shown above. The value here (3000) is
enough to trigger the issue on my x86_64 Linux machine.

Reported-by: Ben Stav <benstav@miggo.io>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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