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.

---

 git-gui/git-gui.sh |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

index 3c0ce26..f727111 100755
@@ -1132,6 +1132,7 @@ 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

1.6.5.rc2.5.gbbf1