git-gui: Create Windows shortcut icons for git-gui.

git-gui: Create Windows shortcut icons for git-gui.

If we are running on Windows we now offer a 'Create Desktop Icon' menu
item under the Project menu.  This pops up a save dialog box letting
the user create a .bat file on their desktop (or somewhere else).  The
.bat script will startup Cygwin with a login shell then launch git-gui
in the current working directory.

This is very useful for Windows users who have little to no desire to
start a command window just to run a git-gui session.

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