Hi all:

Recently I have been playing with the new completion-preview mode and
I'm very happy to see something like this finally added to emacs with a
simple and clean code.

However, there is a missing detail that I find useful when typing:
common preview completion.

The goal is to preview a common prefix when available and longer than
the prefix already inserted. This is a point in between 1) exact match
with one candidate only and 2) getting the first candidate in the whole
list.

I tried to avoid adding complexity to the code (because the thing I like
the most in the package is actually its simplicity and predictability);
but the extra functionality I think worth the few new lines.

I attach the patch for a first review.

Best,
Ergus