Fix the affinity of the "pdiff_required" value.
When using --revision-range with git, we determine whether or not we need to

create a parent diff by checking if the upstream branch contains the source

revision. The result of this check went into a variable called

pdiff_required, but the value of this variable was truthy if a parent diff is

not required. I've negated the assignment expression to flip this back to

mean what it says.

Posted a review request with --revision-range that uses a parent diff.