argv-array: use size_t for count and alloc

On most 64-bit platforms, "int" is significantly smaller than a size_t, which could lead to integer overflow and under-allocation of the array. It's probably impossible to trigger in practice, as it would imply on the order of 2^32 individual allocations. Even if was possible to grow an array in that way (and we typically only use it for sets of strings, like command line options), each allocation needs a pointer, malloc overhead, etc. You'd quite likely run out of RAM before succeeding in such an overflow. But all that hand-waving aside, it's easy enough to use the correct type, so let's do so. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>