dir.c: skip .gitignore, etc larger than INT_MAX

We use add_patterns() to read .gitignore, .git/info/exclude, etc, as well as other pattern-like files like sparse-checkout. The parser for these uses an "int" as an index, meaning that files over 2GB will generally cause signed integer overflow and out-of-bounds access. This is unlikely to happen in any real files, but we do read .gitignore files from the tree. A malicious tree could cause an out-of-bounds read and segfault (we also write NULs over newlines, so in theory it could be an out-of-bounds write, too, but as we go char-by-char, the first thing that happens is trying to read a negative 2GB offset). We could fix the most obvious issue by replacing one "int" with a "size_t". But there are tons of "int" sprinkled throughout this code for things like pattern lengths, number of patterns, and so on. Since nobody would actually want a 2GB .gitignore file, an easy defensive measure is to just refuse to parse them. The "int" in question is in add_patterns_from_buffer(), so we could catch it there. But by putting the checks in its two callers, we can produce more useful error messages. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>