alloc: add repository argument to alloc_tree_node

This is a small mechanical change; it doesn't change the
implementation to handle repositories other than the_repository yet.
Use a macro to catch callers passing a repository other than
the_repository at compile time.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

alloc.c

@@ -57,7 +57,7 @@ void *alloc_blob_node_the_repository(void)
 }
 
 static struct alloc_state tree_state;
-void *alloc_tree_node(void)
+void *alloc_tree_node_the_repository(void)
 {
 	struct tree *t = alloc_node(&tree_state, sizeof(struct tree));
 	t->object.type = OBJ_TREE;