Generates non-reproducible Installed-Size field

[guillemj]

[ Stumbled over this, and did a quick review over the implementation for conformance issues. :) ]

The generated Installed-Size field appears to be created using an out of date algorithm, which produces unreproducible values depending on the file system type and settings.

The current algorithm used by dpkg-gencontrol is described in the deb-substvars(5) man page, the Debian policy is currently out of date, see the following Debian policy bug for the reference to the discussion that prompted the change.