bug#12792: 24.2.50; shell-mode renders all its output in a yellow face

[Eli Zaretskii]

> From: Wolfgang Jenkner <wjenkner@inode.at>
> Date: Sat, 03 Nov 2012 16:37:59 +0100
> Cc: 12792@debbugs.gnu.org
> 
> In bug#11883 you give this example
> 
> echo -e "\033[31mtest\033[39m"
> 
> but ansi-color has never supported SGR parameter 39.  However, before my
> change (the latter commit above), any unknown parameter would have the
> same effect as parameter 0 (turn off all rendition aspects), while it is
> simply ignored now (well, except for an "Invalid face reference: nil"
> message), see `ansi-color-apply-sequence'.  Perhaps, this explains the
> problem.

SGR parameter 39 means go back to the default foreground color.  How
can ansi-color not support it and still be useful?  IIUC, what the old
code did was exactly the right thing (or at least close, since 0 means
turn off _all_ attributes, not just the foreground color), so removing
it without replacing it with something that restores the default
foreground would be a bug, IMO.