commit_list_insert: return the new commit list entry

This is useful for when we want to insert the next one after
this new one, for example.

commit.c

@@ -111,12 +111,13 @@ int parse_commit(struct commit *item)
 	return ret;
 }
 
-void commit_list_insert(struct commit *item, struct commit_list **list_p)
+struct commit_list *commit_list_insert(struct commit *item, struct commit_list **list_p)
 {
 	struct commit_list *new_list = xmalloc(sizeof(struct commit_list));
 	new_list->item = item;
 	new_list->next = *list_p;
 	*list_p = new_list;
+	return new_list;
 }
 
 void free_commit_list(struct commit_list *list)

commit.h

@@ -26,7 +26,7 @@ int parse_commit_buffer(struct commit *item, void *buffer, unsigned long size);
 
 int parse_commit(struct commit *item);
 
-void commit_list_insert(struct commit *item, struct commit_list **list_p);
+struct commit_list * commit_list_insert(struct commit *item, struct commit_list **list_p);
 
 void free_commit_list(struct commit_list *list);