Need high level API for Glacier multipart-upload

Glacier supports multipart uploads just like S3, and do the following steps.
1. initiate multipart-upload
2. multipart uploads
3. complete-multipart-upload

The hard part is #3.
`glacier complete-multipart-upload` command requires `--checksum` which is the SHA256 tree hash of the entire archive, but there is no easy way to compute this checksum with shell scripts.
http://docs.aws.amazon.com/cli/latest/reference/glacier/complete-multipart-upload.html

It would be great if `glacier` has high level `cp` command and

```
if FILESIZE < MULTIPART_THRESHOLD:
basic upload-archive
else:
multipart-upload
```

As you know, `s3 cp` command works this way(8MB threshold is used there).