git-gui: Ensure windows shortcuts always have .bat extension

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 <spearce@spearce.org>