git-gui: Windows tk_getSaveFile is not useful for shortcuts
git-gui invokes the tk_getSaveFile dialog to determine the full path-name of the shortcut file to create. But, on Windows, this dialog always dereferences a shortcut (.lnk) file, as this is essentially a soft-link to its target. If the shortcut file already exists, the dialog returns the path-name of the target (i.e., GIT/cmd/git-gui.exe), and not the desired shortcut file selected by the user. There is no Windows file chooser available in Tcl/Tk that does not dereference .lnk files, so this patch avoids using a dialog: the shortcut to be created is on the desktop and named as "Git + Repository Name". If this .lnk file already exists, the user must give permission to overwrite it or the process terminates. Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>