git-gui: Hide the ugly bash command line from the windows desktop icon.

The user really doesn't need to see the technical details of how we
launch git-gui from within their "desktop icon".  Instead we should hide
the command line from being displayed when the icon launches by putting
@ at the start of the line.  If they really need to see the command we
are running they can edit the batch file.

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