gitk: Fix diffing committed -> staged (typo in diffcmd)

When highlighting a commit, using the context menu over the staged changes
and then selecting "Diff this -> selected" the diff was empty.  The same
happened when highlighting the staged changes and using "Diff selected ->
this" over a commit.  The reason was a copy/paste error in [diffcmd].
This fixes it.
Signed-off-by: NJens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -7225,7 +7225,7 @@ proc diffcmd {ids flags} {
 	set cmd [concat | git diff-index --cached $flags]
 	if {[llength $ids] > 1} {
 	    # comparing index with specific revision
-	    if {$i == 0} {
+	    if {$j == 0} {
 		lappend cmd -R [lindex $ids 1]
 	    } else {
 		lappend cmd [lindex $ids 0]