bug#23129: 25.1.50; Prefix key is not echoed during minibuffer completion

[Drew Adams]

Having just noticed the merge message for bugs 22825 and
23240, and looking at the bug descriptions, it seems to me
that both of them are essentially the same bug as this one,
23129, although this one is echoing during minibuffer input
and that one is about input during key reading.  (That one
is not about just `C-h c'.) 

And really the problem is more general than either of these
bug descriptions.  It is not just about prefix-key echoing
being dropped during minibuffer completion.  And it is not
just about it being dropped during key reading.

It is about prefix-key echoing being dropped when expecting
some input from the user.  The user has hit a prefix key
and that is not getting echoed.  The user gets no feedback.

I recognize that minibuffer reading of input is different,
in terms of implementation, from key reading (reading an
event).  But the user need is the same in these cases:
a prefix key should be echoed, so the user knows what
key s?he hit and knows what Emacs understood and expects.