git-gui: Use Henrik Nyh's git logo icon on Windows systems

Rather than displaying the stock red "Tk" icon in our window
title bars and on the task bar we now show a Git specific logo.
This is Henrik Nyh's logo that we also use in the startup wizard,
scaled to a 16x16 image for Windows task bar usage with a proper
transparent background.

Signed-off-by: Shawn O. Pearce <shawn.o.pearce@bankofamerica.com>
diff --git a/git-gui.sh b/git-gui.sh
index 37da8fd..843191a 100755
--- a/git-gui.sh
+++ b/git-gui.sh
@@ -527,6 +527,10 @@
 	set root_exists 1
 }
 
+if {[is_Windows]} {
+	wm iconbitmap . -default $oguilib/git-gui.ico
+}
+
 ######################################################################
 ##
 ## config defaults