gitk: Fix a bug in make_disporder

The make_disporder function has an optimization where it assumed that if displayorder was already long enough and the first entry in it for a particular arc was non-null, then the whole arc was present. This turns out not to be true in some circumstances, since we can add a commit to an arc (which truncates displayorder to the previous end of that arc), then call make_disporder for later arcs (which will pad displayorder with null elements), then call make_disporder for the first arc - which won't update the null elements. This fixes it by changing the optimization to check the last element for the arc instead of the first. Signed-off-by: Paul Mackerras <paulus@samba.org>