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

submodule.c

@@ -587,7 +587,7 @@ void show_submodule_inline_diff(struct diff_options *o, const char *path,
 		struct object_id *one, struct object_id *two,
 		unsigned dirty_submodule)
 {
-	const struct object_id *old = &empty_tree_oid, *new = &empty_tree_oid;
+	const struct object_id *old = the_hash_algo->empty_tree, *new = the_hash_algo->empty_tree;
 	struct commit *left = NULL, *right = NULL;
 	struct commit_list *merge_bases = NULL;
 	struct child_process cp = CHILD_PROCESS_INIT;