Merge branch 'bc/hash-algo'

An infrastructure to define what hash function is used in Git is
introduced, and an effort to plumb that throughout various
codepaths has been started.

* bc/hash-algo:
  repository: fix a sparse 'using integer as NULL pointer' warning
  Switch empty tree and blob lookups to use hash abstraction
  Integrate hash algorithm support with repo setup
  Add structure representing hash algorithm
  setup: expose enumerated repo info

builtin/am.c

@@ -1433,7 +1433,7 @@ static void write_index_patch(const struct am_state *state)
 	if (!get_oid_tree("HEAD", &head))
 		tree = lookup_tree(&head);
 	else
-		tree = lookup_tree(&empty_tree_oid);
+		tree = lookup_tree(the_hash_algo->empty_tree);
 
 	fp = xfopen(am_path(state, "patch"), "w");
 	init_revisions(&rev_info, NULL);