bug#34622: Find file prompt should indicate sole completion by appending a blank

[積丹尼 Dan Jacobson]

With emacs, one can never be sure if the expansion
Find file or URL: ~/bin/wwzz
is the sole completion or not, without needing a second TAB press to get
"[Sole completion]" shown.

With bash, there is always a space added at end,
"$ wc ~/bin/wwzz "
so we know right away.

I propose that emacs do the same:
"Find file or URL: ~/bin/wwzz "
and if they really want to enter a file with a " " at the end, well they
could use C-q (quoted-insert).

See also 
https://www.gnu.org/software/emacs/manual/html_node/elisp/Minibuffer-Completion.html
but I am specifically talking about C-x C-f (find-file) and C-x d
(dired). Sorry we did discuss this before...