gitk: Fix a bug in make_disporder

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>