[plan] Add diffutils to the backline plan.
[plan] Add diffutils to the backline plan.

The backline Plan provides the interactive environment for our default
Studio type and having `diff` on PATH is really helpful. The driving
desire for this additon was in the creation of a patch file, which is
usually produced with some combination of
`diff -ur <ORIG_DIR> <CHANGED_DIR>`.

georgemarshall/habitat,rsertelon/habitat,rsertelon/habitat,nathenharvey/habitat,nathenharvey/habitat,habitat-sh/habitat,habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,rsertelon/habitat,georgemarshall/habitat,habitat-sh/habitat,nathenharvey/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,rsertelon/habitat,georgemarshall/habitat,rsertelon/habitat,georgemarshall/habitat,nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,georgemarshall/habitat,nathenharvey/habitat,georgemarshall/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat