Prompt percent completion for truncations

[Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>]

This is cute but unhelpful:

[         -- truncation from who knows where

Furthermore, completion after that doesn't work right:

% PS2=%[
parameter
(list of parameter names elided)
% PS2=%\[
(literally nothing: no suggestions, no beep)

If we're going to include %[ in the completions, it should continue
with %[< or %[> ... but my vote would be to just remove that because
it's a deprecated form of %< and %> respectively.  There is also this:

% PS2=%\<
replacement string
% PS2=%\[\<
number

The latter is wrong, of course; both of these should behave the same.