gitk: Don't try to show local changes from a head that isn't shown

gitk: Don't try to show local changes from a head that isn't shown

When updating the display, if the checked-out head has moved on and
isn't currently shown, and there are local changes, we could try to
insert a fake row with a parent that isn't displayed, leading to a
Tcl error.  This is because we check whether the checked-out head
is displayed before rereading the references (which is when we discover
that the head has moved).  This fixes it.

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