Restore reads a tar archive from r and extracts all of its files into dir, using only the base name of each file.

Generates a filtering function for Stream that checks an incoming file, and only writes the file to the stream if its mod time is later than since.

Stream is a convenience function for creating a tar of a shard dir.

stream a single file to tw, extending the header name using the shardRelativePath.

Stream a single file to tw, using tarHeaderFileName instead of the actual filename e.g., when we want to write a *.tmp file using the original file's non-tmp name.

Untar takes a destination path and a reader; a tar reader loops over the tarfile creating the file structure at 'dir' along the way, and writing any files.