gitk: Work around empty back and forward images when buttons are disabled

gitk: Work around empty back and forward images when buttons are disabled

On Mac, the back and forward buttons show an empty rectange instead of
a grayed-out arrow when they are disabled. The reason is a Tk bug on Mac
that causes disabled images not to draw correctly (not to draw at all,
that is); see
<https://groups.google.com/forum/?fromgroups=#!topic/comp.lang.tcl/V-nW1JBq0eU>.

To work around this, we explicitly provide gray images for the disabled
state; I think this looks better than the default stipple effect that you
get on Windows as well, but that may be a matter of taste.

Signed-off-by: Stefan Haller <stefan@haller-berlin.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>