First line: --from
Second line: --to
Numbers: revision height, greater means newer


1 2 3                      > forward
       4 5 6               >

1 2 3                      > forward
    3 4 5                  >

1 2 3                      > forward
  2 3 4                    >

1 2 3                      > forward
1 2 3 4                    >

1 2 3                      > log these revs just like --revision would do,
1 2 3                      > ie. look at --next / --last

1 2 X 4                    > merge the sets and log just like --revision would do
1 Y 3 4                    > ie. look at --next / --last

1 2 3 4                    > backward
1 2 3                      >

  2 3 4                    > backward
1 2 3                      >

    3 4 5                  > backward
1 2 3                      >

      4 5 6                > backward
1 2 3                      >


pseudo algorithm:

min_height(set) = height(take_first(toposort(set)))
max_height(set) = height(take_last(toposort(set)))

if (min_height(__from) <= min_height(__to) &&
    max_height(__from) <  max_height(__to))
    direction = forward
else
if (min_height(__from) >= min_height(__to) &&
    max_height(__from) >  max_height(__to))
    direction = backward;
else
    direction = next >= 0 ? forward : backward;