git-gui: fix use of uninitialized variable

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>

git-gui.sh

@@ -1134,6 +1134,7 @@ set current_branch {}
 set is_detached 0
 set current_diff_path {}
 set is_3way_diff 0
+set is_submodule_diff 0
 set is_conflict_diff 0
 set selected_commit_type new
 set diff_empty_count 0