[PATCH 3/5] reftable/stack: fix compiler warning due to missing braces

[Patrick Steinhardt <ps@xxxxxx>]

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 <ps@xxxxxx>
---
 reftable/stack.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/reftable/stack.c b/reftable/stack.c
index 4caf96aa1d..3480ad21c3 100644
--- a/reftable/stack.c
+++ b/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,

-- 
2.50.1