alloc: add repository argument to alloc_commit_index

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

@@ -82,7 +82,7 @@ void *alloc_object_node_the_repository(void)
 
 static struct alloc_state commit_state;
 
-unsigned int alloc_commit_index(void)
+unsigned int alloc_commit_index_the_repository(void)
 {
 	static unsigned int count;
 	return count++;
@@ -92,7 +92,7 @@ void *alloc_commit_node_the_repository(void)
 {
 	struct commit *c = alloc_node(&commit_state, sizeof(struct commit));
 	c->object.type = OBJ_COMMIT;
-	c->index = alloc_commit_index();
+	c->index = alloc_commit_index(the_repository);
 	return c;
 }