bug#19031: 24.4; find-file in icomplete-mode shows completions with no input

[Juri Linkov <juri@linkov.net>]

> I originally reported this because I found it jarring to get a bunch
> of completions without having entered anything. In my home dir it
> basically shows me garbage (dot files that I'm never interested in).
>
> Would it not be possible to make a difference between the case where
> find-file provides some default text (current dir) and where I have
> entered some input? The variable literally says -on-no-input.

I tried to handle this case with the following patch, but it doesn't work
because read-file-name-default resets `minibuffer-default' to nil,
so icomplete doesn't know what was initial input in the minibuffer.

diff --git a/lisp/icomplete.el b/lisp/icomplete.el
index 676917b9da..47bd8f90a4 100644
--- a/lisp/icomplete.el
+++ b/lisp/icomplete.el
@@ -579,7 +579,10 @@ icomplete-exhibit
         (goto-char (point-max))
                                         ; Insert the match-status information:
         (when (and (or icomplete-show-matches-on-no-input
-                       (> (icomplete--field-end) (icomplete--field-beg)))
+                       (if (stringp minibuffer-default)
+                           (/= (icomplete--field-end) (+ (icomplete--field-beg)
+                                                         (length minibuffer-default)))
+                         (> (icomplete--field-end) (icomplete--field-beg))))
                    (or
                     ;; Don't bother with delay after certain number of chars:
                     (> (- (point) (icomplete--field-beg))