argv-array: refactor empty_argv initialization

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>