git-gui: Improve the icons used in the browser display.

git-gui: Improve the icons used in the browser display.

Real icons which seem to indicate going up to the parent (an up arrow)
and a subdirectory (an open folder).  Files are now drawn with the
file_mod icon, like a modified file is.  This just looks better as it
is more consistent with the rest of our UI.

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