The tar files we're making of artifacts don't include parent
directories. Now we could change the builder to make them, but we can
also just autocreate them on import. Mode 0755 with no xattrs seems
OK here.
It's pretty trivial to map a previously existing commit tree into a
mutable tree too. While we're here change the command line arguments
for commit so that we can now properly overlay any combination of
directory, commit, or tarfile.
Colin Walters