diff(1): don't reject specifying the same format multiple times

This may happen, for instance, if one happens to have an alias of diff to
diff -up and attempts to specify the amount of context on top of that.

Aliases like this may cause other problems, but if they're really not ever
generating non-unified diffs then we should at least not break that
use-case.

In addition, we'll now pick up a format mismatch if -p is specified with
!contextual && !unified && !unset.

Fix up a small trailing whitespace nit in the tests while we're here, and
add tests to make sure that we can double up all the formatting options.

Reported by: jbeich
MFC after: 3 days