dir.c: reduce max pattern file size to 100MB

dir.c: reduce max pattern file size to 100MB

In a2bc523e1e (dir.c: skip .gitignore, etc larger than INT_MAX,
2024-05-31) we put capped the size of some files whose parsing code and
data structures used ints. Setting the limit to INT_MAX was a natural
spot, since we know the parsing code would misbehave above that.

But it also leaves the possibility of overflow errors when we multiply
that limit to allocate memory. For instance, a file consisting only of
"a\na\n..." could have INT_MAX/2 entries. Allocating an array of
pointers for each would need INT_MAX*4 bytes on a 64-bit system, enough
to overflow a 32-bit int.

So let's give ourselves a bit more safety margin by giving a much
smaller limit. The size 100MB is somewhat arbitrary, but is based on the
similar value for attribute files added by 3c50032ff5 (attr: ignore
overly large gitattributes files, 2022-12-01).

There's no particular reason these have to be the same, but the idea is
that they are in the ballpark of "so huge that nobody would care, but
small enough to avoid malicious overflow". So lacking a better guess, it
makes sense to use the same value. The implementation here doesn't share
the same constant, but we could change that later (or even give it a
runtime config knob, though nobody has complained yet about the
attribute limit).

And likewise, let's add a few tests that exercise the limits, based on
the attr ones. In this case, though, we never read .gitignore from the
index; the blob code is exercised only for sparse filters. So we'll
trigger it that way.

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