bug#13875: 24.3.50; whitespace-mode should use 'prepend for whitespace-line face

[Dmitry Gutov]

Tags: patch

Right now, there's no way to tell whitespace-mode to keep the foreground
color of the underlying text when highlighting lines that are too long.

Related SO question:
http://stackoverflow.com/questions/14636786/how-to-unset-the-foreground-color-of-whitespace-mode-for-emacs

With the patch at the bottom applied, the following code will make it
keep the text's foreground and just change background:

  (set-face-attribute 'whitespace-line nil :foreground nil
                                           :background "gainsboro")

(Changing :backround is not necessary, but gray20 looks bad with the
default foreground color.)

=== modified file 'lisp/whitespace.el'
--- lisp/whitespace.el  2013-01-11 23:08:55 +0000
+++ lisp/whitespace.el  2013-03-05 16:25:51 +0000
@@ -2291,7 +2291,7 @@
         (if (memq 'lines whitespace-active-style)
             0                          ; whole line
           2)                           ; line tail
-        whitespace-line t))
+        whitespace-line 'prepend))
        t))
     (cond
      ((memq 'space-before-tab whitespace-active-style)