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-atom.c b/ui-atom.c
index cd66f82..1056f36 100644
--- a/ui-atom.c
+++ b/ui-atom.c
@@ -140,7 +140,7 @@
 	}
 	while ((commit = get_revision(&rev)) != NULL) {
 		add_entry(commit, host);
-		free_commit_buffer(commit);
+		free_commit_buffer(the_repository->parsed_objects, commit);
 		free_commit_list(commit->parents);
 		commit->parents = NULL;
 	}