*.[ch] *_INIT macros: use { 0 } for a "zero out" idiom

*.[ch] *_INIT macros: use { 0 } for a "zero out" idiom

In C it isn't required to specify that all members of a struct are
zero'd out to 0, NULL or '\0', just providing a "{ 0 }" will
accomplish that.

Let's also change code that provided N zero'd fields to just
provide one, and change e.g. "{ NULL }" to "{ 0 }" for
consistency. I.e. even if the first member is a pointer let's use "0"
instead of "NULL". The point of using "0" consistently is to pick one,
and to not have the reader wonder why we're not using the same pattern
everywhere.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>