diffs: cleanup of variable naming around cut_off_limit

A brief summary of this area:

The base controller sets self.cut_off_limit from config and is used for diffs,
unless controllers are given a fulldiff query parameter. In a few cases, these
are passed to templates as c.cut_off_limit or c.fulldiff . Also, if the diff
function returns a LimitedDiffContainer, c.limited_diff is set so the UI can
report the data set is partial.