Navigation: When going forward, grab transition from current entry

Our history entries contain exactly one transition: The one used to arrive at
the entry. In addition, the index of the current entry and that of the previous
entry is updated before the value for the transition is computed. So, it makes
sense, when going backwards, to read the transition value from the previous
entry, because that value stores the transition that was used to get there from
where history is currently at.

However, when going forward, it makes no sense to read the transition value
from the previous entry, because that one stores the transition that was used
to reach it, not the transition that must be used to reach the next entry.

Fixes gh-1867