The reftable ref store needs to keep track of multiple stacks, one for
the main worktree and an arbitrary number of stacks for worktrees. This
is done by storing pointers to `struct reftable_stack`, which we then
access directly.

Wrap the stack in a new `struct reftable_backend`. This will allow us to
attach more data to each respective stack in subsequent commits.

Signed-off-by: Patrick Steinhardt <redacted>
---
 refs/reftable-backend.c | 129 +++++++++++++++++++++++-----------------
 1 file changed, 73 insertions(+), 56 deletions(-)

diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index 38eb14d591..43cba53cb1 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c

@@ -32,6 +32,23 @@
  */
 #define REF_UPDATE_VIA_HEAD (1 << 8)

+struct reftable_backend {
+	struct reftable_stack *stack;
+};
+
+static int reftable_backend_init(struct reftable_backend *be,
+				 const char *path,
+				 const struct reftable_write_options *opts)
+{
+	return reftable_new_stack(&be->stack, path, opts);
+}
+
+static void reftable_backend_release(struct reftable_backend *be)
+{
+	reftable_stack_destroy(be->stack);
+	be->stack = NULL;
+}
+
 struct reftable_ref_store {
 	struct ref_store base;

@@ -39,17 +56,17 @@ struct reftable_ref_store {
 	 * The main stack refers to the common dir and thus contains common
 	 * refs as well as refs of the main repository.
 	 */

@@ -772,14 +789,14 @@ static struct ref_iterator *reftable_be_iterator_begin(struct ref_store *ref_sto
 	 * right now. If we aren't, then we return the common reftable
 	 * iterator, only.
 	 */
-	 if (!refs->worktree_stack)
+	 if (!refs->worktree_backend.stack)