gitk: Get rid of the rowchk array

gitk: Get rid of the rowchk array

Instead, when looking for lines that should be terminated with a down
arrow, we look at the parents of the commit $downarrowlen + 1 rows
before.  This gets rid of one more place where we are assuming that
all the rows are laid out in order from top to bottom.

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