[PATCH v2 3/6] reftable/stack: fix compiler warning due to missing braces

While perfectly legal, older compiler toolchains complain when zero-initializing structs that contain nested structs with `{0}`:

    /home/libgit2/source/deps/reftable/stack.c:862:35: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
            struct reftable_addition empty = REFTABLE_ADDITION_INIT;
                                             ^~~~~~~~~~~~~~~~~~~~~~
    /home/libgit2/source/deps/reftable/stack.c:707:33: note: expanded from macro 'REFTABLE_ADDITION_INIT'
    #define REFTABLE_ADDITION_INIT {0}
                                    ^

Silence this warning by using `{{0}}` instead.

Note that we had the discussion around whether or not we want to handle such errors in the past already [1], where we basically decided that we do not care about such old-and-buggy compilers. But the reftable library is a special case because it is used by projects other than Git, and libgit2 for example hits the above issue in its pipeline. As there is only a single problematic instance of this issue we do the pragmatic thing and simply make the compiler happy.

[1]: https://lore.kernel.org/git/20220710081135.74964-1-sunshine@sunshineco.com/T/

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 reftable/stack.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)