tree: add repository argument to lookup_tree

Add a repository argument to allow the callers of lookup_tree
to be more specific about which repository to act on. This is a small
mechanical change; it doesn't change the implementation to handle
repositories other than the_repository yet.

As with the previous commits, use a macro to catch callers passing a
repository other than the_repository at compile time.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/am.c b/builtin/am.c
index 6273ea5..72e928c 100644
--- a/builtin/am.c
+++ b/builtin/am.c
@@ -32,6 +32,7 @@
 #include "apply.h"
 #include "string-list.h"
 #include "packfile.h"
+#include "repository.h"
 
 /**
  * Returns 1 if the file is empty or does not exist, 0 otherwise.
@@ -1400,9 +1401,10 @@ static void write_index_patch(const struct am_state *state)
 	FILE *fp;
 
 	if (!get_oid_tree("HEAD", &head))
-		tree = lookup_tree(&head);
+		tree = lookup_tree(the_repository, &head);
 	else
-		tree = lookup_tree(the_hash_algo->empty_tree);
+		tree = lookup_tree(the_repository,
+				   the_repository->hash_algo->empty_tree);
 
 	fp = xfopen(am_path(state, "patch"), "w");
 	init_revisions(&rev_info, NULL);