git-gui: if a background colour is set, set foreground colour as well

In several places, only the background colour is set to an explicit
value, sometimes even "white".  This does not work well with dark
colour themes.

This patch tries to set the foreground colour to "black" in those
situations, where an explicit background colour is set without defining
any foreground colour.

Signed-off-by: Philipp A. Hartmann <ph@sorgh.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

lib/error.tcl

@@ -80,7 +80,9 @@ proc hook_failed_popup {hook msg {is_fatal 1}} {
 		-justify left \
 		-font font_uibold
 	text $w.m.t \
-		-background white -borderwidth 1 \
+		-background white \
+		-foreground black \
+		-borderwidth 1 \
 		-relief sunken \
 		-width 80 -height 10 \
 		-font font_diff \