gitk: Fix two bugs reported by users

gitk: Fix two bugs reported by users

The first was a simple typo where I put $yc instead of [yc $row].
The second was that I broke the logic for keeping up with fast
movement through the commits, e.g. when you select a commit and then
press down-arrow and let it autorepeat.  That got broken when I
changed the merge diff display to use git-diff-tree --cc.

Signed-off-by: Paul Mackerras <paulus@samba.org>