PDL: tell ‘diff’ to output a colored diff

This makes it much easier to see the added and removed lines in the
logs. Use ‘less -R’ when viewing a log file to allow the escape
sequences to show up on the terminal.

Test: a provoked error shows a colorful diff: http://screen/bEf5puAqZGVbC5w
Change-Id: I3318015ec563e47be5317298464ef527d1809424