argv-array: refactor empty_argv initialization

An empty argv-array is initialized to point to a static empty NULL-terminated array. The original implementation separates the actual storage of the NULL-terminator from the pointer to the list. This makes the exposed type a "const char **", which nicely matches the type stored by the argv-array. However, this indirection means that one cannot use empty_argv to initialize a static variable, since it is not a constant. Instead, we can expose empty_argv directly, as an array of pointers. The only place we use it is in the ARGV_ARRAY_INIT initializer, and it decays to a pointer appropriately there. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>