reftable/merged: expose functions to initialize iterators

We do not expose any functions via our public headers that would allow a
caller to initialize a reftable iterator from a merged table. Instead,
they are expected to go via the generic `reftable_table` interface,
which is somewhat roundabout.

Implement two new functions to initialize iterators for ref and log
records to plug this gap.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/merged.c

@@ -254,6 +254,18 @@ void merged_table_init_iter(struct reftable_merged_table *mt,
 	iterator_from_merged_iter(it, mi);
 }
 
+void reftable_merged_table_init_ref_iterator(struct reftable_merged_table *mt,
+					     struct reftable_iterator *it)
+{
+	merged_table_init_iter(mt, it, BLOCK_TYPE_REF);
+}
+
+void reftable_merged_table_init_log_iterator(struct reftable_merged_table *mt,
+					     struct reftable_iterator *it)
+{
+	merged_table_init_iter(mt, it, BLOCK_TYPE_LOG);
+}
+
 uint32_t reftable_merged_table_hash_id(struct reftable_merged_table *mt)
 {
 	return mt->hash_id;

reftable/reftable-merged.h

@@ -36,6 +36,14 @@ int reftable_new_merged_table(struct reftable_merged_table **dest,
 			      struct reftable_table *stack, size_t n,
 			      uint32_t hash_id);
 
+/* Initialize a merged table iterator for reading refs. */
+void reftable_merged_table_init_ref_iterator(struct reftable_merged_table *mt,
+					     struct reftable_iterator *it);
+
+/* Initialize a merged table iterator for reading logs. */
+void reftable_merged_table_init_log_iterator(struct reftable_merged_table *mt,
+					     struct reftable_iterator *it);
+
 /* returns the max update_index covered by this merged table. */
 uint64_t
 reftable_merged_table_max_update_index(struct reftable_merged_table *mt);