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/merge-recursive.c b/merge-recursive.c
index 63bc0e2..fbb044e 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -158,7 +158,7 @@ static struct tree *shift_tree_object(struct tree *one, struct tree *two,
 	}
 	if (!oidcmp(&two->object.oid, &shifted))
 		return two;
-	return lookup_tree(&shifted);
+	return lookup_tree(the_repository, &shifted);
 }
 
 static struct commit *make_virtual_commit(struct tree *tree, const char *comment)
@@ -416,7 +416,7 @@ struct tree *write_tree_from_memory(struct merge_options *o)
 		return NULL;
 	}
 
-	result = lookup_tree(&active_cache_tree->oid);
+	result = lookup_tree(the_repository, &active_cache_tree->oid);
 
 	return result;
 }
@@ -3405,7 +3405,7 @@ int merge_recursive(struct merge_options *o,
 		/* if there is no common ancestor, use an empty tree */
 		struct tree *tree;
 
-		tree = lookup_tree(the_hash_algo->empty_tree);
+		tree = lookup_tree(the_repository, the_repository->hash_algo->empty_tree);
 		merged_common_ancestors = make_virtual_commit(tree, "ancestor");
 	}