git-gui: Make the tree browser also use lightgray selection

git-gui: Make the tree browser also use lightgray selection

In 9adccb05 Matthijs Melchior changed our selection colors in the
main index/working directory file lists to use a lightgray as the
background color as this made the UI easier to read on all platforms.

When we did that change we missed doing also doing in the file
browser UI.  Doing so just makes the entire thing UI consistent.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>