Comment 5 for bug 1725829

With this patch, duplicity at least wrote the full .sigtar.gz file
(2630787933 bytes), and reading it with "tar tvfz" produces no error.
I did not yet try to make an incremental updated on top of it.

(Unfortunately, block_iter does not define the .__iter__() method, so
I could not simply write "for block in block_iter".)