In 2017, GNU diff had been replaced by BSD licensed diff in our base,
but some features of GNU diff had not been implemented, particularly,
the ``group format'' family of options. Instead of checking for the
${OSVERSION} value, check if installed diff(1) implementation is GNU
or BSD one based on the --version output.
Because conditions are subject to immediate expansion, we cannot use
${DIFF} and ${GREP} variables; however, literal ``diff'' is actually
correct in this particular case as user's preferred diff(1) does not
necessarily point to ``/usr/bin/diff'' which ${DIFF} expands to.
While here, amend the EDITOR option description so it clearly refers
to the internal *text* editor.
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