git-gui: Ensure windows shortcuts always have .bat extension
Apparently under some setups on Windows Tk is hiding our file extension recommendation of ".bat" from the user and that is allowing the user to create a shortcut file which has no file extension. Double clicking on such a file in Windows Explorer brings up the associate file dialog, as Windows does not know what application to launch.
We now append the file extension ".bat" to the filename of the shortcut file if it has no extension or if it has one but it is not ".bat".
Signed-off-by: Shawn O. Pearce <firstname.lastname@example.org>