git-gui: Windows tk_getSaveFile is not useful for shortcuts

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>