project--completing-read-strict breaks ada-mode project completion table

[Stephen Leake]

Commit 8f9d93f3054 (Dmitry Gutov   2018-12-29 415) breaks my ada-mode
completion table.

That commit modifies the completion table by first calling the actual
completion table to get all the file names, then removing any common
prefix directory, then calling completing read with that file list.

This violates assumptions made within the ada-mode completion table, and
breaks the expected completion style - it requires typing the remaining
directory names first; the ada-mode completion table allows completing
on the base name only.

I can understand the motivation behind removing common prefixes, but the
proper way to do that is to implement a completion style and/or table
that does that, and make that style/table the default for projects.

The ada-mode completion style and table eliminates all directories from
the visible completion string, except the minimum needed to make the
string unique. I think this is a better solution to the problem;
project.el should allow the user to choose their prefered solution, by
choosing a completion style and table.

To make my source available for experimenting with, I'd like to create
several new ELPA packages; I sent a separate email about that.

--
-- Stephe