git: update to v2.21.0

Update to git version v2.21.0. Required changes follow upstream commits:

* 6a7895fd8a3bd409f2b71ffc355d5142172cc2a0
  (commit: prepare free_commit_buffer and release_commit_memory for
  any repo)

* e092073d643b17c82d72cf692fbfaea9c9796f11
  (tree.c: make read_tree*() take 'struct repository *')

Signed-off-by: Christian Hesse <mail@eworm.de>
Reviewed-by: John Keeping <john@keeping.me.uk>
diff --git a/ui-tree.c b/ui-tree.c
index 314ac52..4be89c8 100644
--- a/ui-tree.c
+++ b/ui-tree.c
@@ -185,8 +185,8 @@
 		tree_ctx.name = NULL;
 		tree_ctx.count = 0;
 
-		read_tree_recursive(tree, "", 0, 1, &paths, single_tree_cb,
-				    &tree_ctx);
+		read_tree_recursive(the_repository, tree, "", 0, 1,
+			&paths, single_tree_cb, &tree_ctx);
 
 		if (tree_ctx.count != 1)
 			break;
@@ -294,7 +294,8 @@
 	}
 
 	ls_head();
-	read_tree_recursive(tree, "", 0, 1, &paths, ls_item, walk_tree_ctx);
+	read_tree_recursive(the_repository, tree, "", 0, 1,
+		&paths, ls_item, walk_tree_ctx);
 	ls_tail();
 }
 
@@ -373,7 +374,8 @@
 		goto cleanup;
 	}
 
-	read_tree_recursive(commit->maybe_tree, "", 0, 0, &paths, walk_tree, &walk_tree_ctx);
+	read_tree_recursive(the_repository, commit->maybe_tree, "", 0, 0,
+		&paths, walk_tree, &walk_tree_ctx);
 	if (walk_tree_ctx.state == 1)
 		ls_tail();
 	else if (walk_tree_ctx.state == 2)