[Tails-dev] upgrade vs. random files on source device

Hi,

We've seen such issues at "Clone & upgrade" time reported quite often
recently on our various support channels:

> Not enough free space on device.
> 1712MB ISO + 0MB overlay > 1485MB free space

Every time it was caused by random cruft (generally a Tails ISO) added
to the source USB stick, and every time we were able to communicate
with the reporter long enough, removing that cruft fixed the issue.

First, there seems to be a bug in the 1712MB size computation: even
with random files were added on the source partition, given the size
of the source partition is the same as the size of the target
partition, it has to fit. This should be trivial to fix.

Second, perhaps we could add a very dumb and simple filter to the
clone process, to skip copying .iso files. This would have avoided the
issue in all cases I've been given to see.

Anyone wants to take a look at the liveusb-creator code,
and try to implement the one or the other?

Cheers!