historical: Always format rates with the same precision.

When we format a rate as a price, we don't know how much precision
is "enough" to do the conversion, because we don't know what's
being converted to. As a result, we may (=will almost certainly)
end up formatting the rate with different precision on the cost
date vs. the price date, and that causes Beancount/Ledger to fail
to make the connection between them.

Using a constant of 6 is enough to make the current test for
"enough" precision pass, so just do that for now. This might need
further refinement in the future.