diff --git a/commit.c b/commit.c
index aa3b35b6a8..8f6b703c55 100644
--- a/commit.c
+++ b/commit.c
@@ -316,6 +316,26 @@ int parse_commit(struct commit *item)
 	return ret;
 }
 
+static void unparse_commit_list(struct commit_list *list)
+{
+	for (; list; list = list->next)
+		unparse_commit(list->item);
+}
+
+void unparse_commit(struct commit *item)
+{
+	item->object.flags = 0;
+	item->object.used = 0;
+	if (item->object.parsed) {
+		item->object.parsed = 0;
+		if (item->parents) {
+			unparse_commit_list(item->parents);
+			free_commit_list(item->parents);
+			item->parents = NULL;
+		}
+	}
+}
+
 struct commit_list *commit_list_insert(struct commit *item, struct commit_list **list_p)
 {
 	struct commit_list *new_list = xmalloc(sizeof(struct commit_list));
diff --git a/commit.h b/commit.h
index ba9f63813e..f3eaf1d048 100644
--- a/commit.h
+++ b/commit.h
@@ -40,6 +40,8 @@ int parse_commit_buffer(struct commit *item, void *buffer, unsigned long size);
 
 int parse_commit(struct commit *item);
 
+void unparse_commit(struct commit *item);
+
 struct commit_list * commit_list_insert(struct commit *item, struct commit_list **list_p);
 unsigned commit_list_count(const struct commit_list *l);
 struct commit_list * insert_by_date(struct commit *item, struct commit_list **list);