This fixes a bug introduced by the "display summary when showing diff of a
submodule" patch. It lead to a "no such variable" error when opening the
diff context menu while no diff was shown.
Signed-off-by: NJens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>