git-gui: Use proper Windows shortcuts instead of bat files

git-gui: Use proper Windows shortcuts instead of bat files

On Windows its better to use a shortcut (.lnk file) over a batch
script (.bat) as we can specify the icon file for the .lnk and
thus have these git specific objects appear on the desktop with
that git specific icon file.

Unfortunately the authors of Tcl did not bless us with the APIs
needed to create shortcuts from within Tcl.  But Microsoft did
give us Windows Scripting Host which allows us to execute some
JavaScript that calls some sort of COM object that can operate
on a .lnk file.

We now build both Cygwin and non-Cygwin "desktop icons" as proper
Windows .lnk files, using the "Start in" property of these files
to indicate the working directory of the repository the user wants
to launch.

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