alloc: add repository argument to alloc_commit_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

@@ -88,7 +88,7 @@ unsigned int alloc_commit_index(void)
 	return count++;
 }
 
-void *alloc_commit_node(void)
+void *alloc_commit_node_the_repository(void)
 {
 	struct commit *c = alloc_node(&commit_state, sizeof(struct commit));
 	c->object.type = OBJ_COMMIT;