Previously, if kiss-size was ran on a package that owns a lot of
files, kiss-size would error out. This is because du wasn't
able to handle the number of files provided to it.
This patch solves this issue, by avoiding the generation of the 'files'
variable, and instead piping directly into xargs which then feeds the
files to du.
For an example package that breaks with the previous implementation,
see https://github.com/ehawkvu/kiss-tex/tree/master/texlive/texlive-fontsextra
which installs around 94,000 files.
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