bug#12377: 24.2.50; `minibuffer-prompt-properties' should respect existing faces in prompt string

[Drew Adams <drew.adams@oracle.com>]

> I've now implemented this on the trunk -- the faces are merged with the
> face specified by -prompt-properties (which are at the end), as
> suggested by Stefan.

There was nothing posted to this bug thread by Stefan.  Likewise
for the other bug, #16136, with which this one was merged.

The above post by Lars was in reply to this:

>> Do you have a recipe that exhibits this bug?
> emacs -Q
> (completing-read (propertize "My prompt:" 'face 'highlight) '(a b c d))

Please reopen this bug.  It is NOT fixed.  At least not in this
build from 5/25, which is long after the "fix" was implemented:

In GNU Emacs 25.0.94.1 (x86_64-w64-mingw32)
 of 2016-05-25 built on KAEL
Windowing system distributor 'Microsoft Corp.', version 6.1.7601
Configured using:
 'configure --prefix=/tmp/emacs --without-imagemagick
 --enable-checking=yes,glyphs --enable-check-lisp-object-type
 'CFLAGS=-Og -gdwarf-4 -g3''

You still cannot do what was requested, including the obvious
as illustrated by the above `completing-read' test case.