In scripts -d/--diff, show either all percentages or none

Previously, with -d/--diff, we would only show non-zero percentages. But
this was ambiguous/confusing when dealing with multiple results
(stack.py, summary.py, etc).

To help with this, I've switched to showing all percentages unless all
percentages are zero (no change). This matches the -d/--diff row-hiding
logic, so by default all rows should show all percentages.

Note -p/--percent did not change, as it already showed all percentages
all of the time.

scripts/code.py

@@ -465,8 +465,7 @@ def table(Result, results, diff_results=None, *,
                     '+∞%' if t == +m.inf
                     else '-∞%' if t == -m.inf
                     else '%+.1f%%' % (100*t)
-                    for t in ratios
-                    if t)
+                    for t in ratios)
                 if any(ratios) else '')
         return entry