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.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

reftable/stack.c

@@ -704,7 +704,7 @@ struct reftable_addition {
 	uint64_t next_update_index;
 };
 
-#define REFTABLE_ADDITION_INIT {0}
+#define REFTABLE_ADDITION_INIT {{0}}
 
 static int reftable_stack_init_addition(struct reftable_addition *add,
 					struct reftable_stack *st,