gitk: set uicolor SystemButtonFace error on X11 if .gitk created using Win32 tk

Would you please change gitk to not hard-code Win32-specific color values when creating .gitk on a Win32 windowing system?

Gitk stopped working for me on Cygwin when Cygwin changed from using a Win32 native version of tk to using the standard X11 version. The error was because gitk had previously created a .gitk file using Win32 specific color values:

https://github.com/gitster/git/commit/1924d1bc0dc99cd3460d3551671908cc76 c09d3b

I was able to work around the problem by replacing the Win32 specific
colors in my .gitk file with the default colors gitk uses on other
windowing systems.
 
See also:

http://cygwin.com/ml/cygwin/2012-02/msg00391.html