gitk: Fix bug introduced in commit 67a4f1a7

gitk: Fix bug introduced in commit 67a4f1a7

In fixing the "can't unset idinlist" error, I moved the setting of
idinlist into the loop that splits the parents into "new" parents
(i.e. those of which this is the first child) and "old" parents.
Unfortunately this is incorrect in the case where we hit the break
statement a few lines further down, since when we come back in,
we'll see idinlist($p) set for some parents that aren't in the list.

This fixes it by moving the loop that sets up newolds and oldolds
further down.

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