git-gui: fix use of undeclared variable diff_empty_count

Commit 584fa9cc introduced the global variable diff_empty_count, which
is used in diff.tcl. This variable wasn't declared anywhere which
resulted in an ugly error message box instead of the intended
informative message.
Signed-off-by: NJoerg Bornemann <joerg.bornemann@web.de>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -1108,6 +1108,7 @@ set current_diff_path {}
 set is_3way_diff 0
 set is_conflict_diff 0
 set selected_commit_type new
+set diff_empty_count 0
 
 set nullid "0000000000000000000000000000000000000000"
 set nullid2 "0000000000000000000000000000000000000001"