RCS is still useful[1], at least I used for this.

T=`mktemp -d /tmp/differ-XXXXXXXXX`
cd $T
mkdir RCS
touch f
ci -t-desc f
while true
do
  co -l f
  $* 2>&1 | tr -cd '[:print:]\n' > f
  if ! rcsdiff f > /dev/null 2>&1
  then
    rcsdiff -c 10 f | mail -s "$*"  $USER
  fi
  ci -mc f
  sleep 60
done

[1] some RCS articles I stumbled upon -

gopher://box.matto.nl/0/rcs.txt