refs: export ref_transaction_update_reflog()

In a subsequent commit we'll add another user that wants to write reflog
entries. This requires them to call `ref_transaction_update_reflog()`,
but that function is local to "refs.c".

Export the function to prepare for the change. While at it, drop the
`flags` field, as all callers are for now expected to use the same flags
anyway.

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

refs.h

@@ -795,6 +795,21 @@ int ref_transaction_update(struct ref_transaction *transaction,
 			   unsigned int flags, const char *msg,
 			   struct strbuf *err);
 
+/*
+ * Similar to `ref_transaction_update`, but this function is only for adding
+ * a reflog update. Supports providing custom committer information. The index
+ * field can be utiltized to order updates as desired. When set to zero, the
+ * updates default to being ordered by refname.
+ */
+int ref_transaction_update_reflog(struct ref_transaction *transaction,
+				  const char *refname,
+				  const struct object_id *new_oid,
+				  const struct object_id *old_oid,
+				  const char *committer_info,
+				  const char *msg,
+				  uint64_t index,
+				  struct strbuf *err);
+
 /*
  * Add a reference creation to transaction. new_oid is the value that
  * the reference should have after the update; it must not be