bin/150727: diff on UTF-8 text files thinks they are binary - regression from 7.X

[Pete French]

> I couldn't reproduce this with simple UTF-8 files:

I just looked through my example files in detail, and it turns out the
problem is not with UTF-8 after all, but with NULL characters which
are also in the file. This is what trips up 'diff' - and though it
it a charge from 7.X I am not sure that it is really a bug.

Sorry for the noise - the code I used to verify that the file was
a valid UTF-8 file accepts the zero bytes quite happily and says
that it is a text file.