gitk: Fix "wrong # coordinates" error on reload

gitk: Fix "wrong # coordinates" error on reload

This fixes the Tk error "wrong # coordinates: expected 0 or 4, got 2"
that sometimes occurred when reloading.  The problem was that we didn't
unset the variables containing the canvas item id numbers for the
displayed rows when we cleared the canvases.  Thus make_secsel would
think it had something to do when it didn't.

Thanks to Michele Ballabio for finding a way to trigger the bug
reliably.

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