reftable/generic: adapt interface to allow reuse of iterators

Refactor the interfaces exposed by `struct reftable_table` and `struct
reftable_iterator` such that they support iterator reuse. This is done
by separating initialization of the iterator and seeking on it.

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

reftable/reftable-iterator.h

@@ -21,12 +21,33 @@ struct reftable_iterator {
 	void *iter_arg;
 };
 
+/*
+ * Position the iterator at the ref record with given name such that the next
+ * call to `next_ref()` would yield the record.
+ */
+int reftable_iterator_seek_ref(struct reftable_iterator *it,
+			       const char *name);
+
 /* reads the next reftable_ref_record. Returns < 0 for error, 0 for OK and > 0:
  * end of iteration.
  */
 int reftable_iterator_next_ref(struct reftable_iterator *it,
 			       struct reftable_ref_record *ref);
 
+/*
+ * Position the iterator at the log record with given name and update index
+ * such that the next call to `next_log()` would yield the record.
+ */
+int reftable_iterator_seek_log_at(struct reftable_iterator *it,
+				  const char *name, uint64_t update_index);
+
+/*
+ * Position the iterator at the newest log record with given name such that the
+ * next call to `next_log()` would yield the record.
+ */
+int reftable_iterator_seek_log(struct reftable_iterator *it,
+			       const char *name);
+
 /* reads the next reftable_log_record. Returns < 0 for error, 0 for OK and > 0:
  * end of iteration.
  */