76dfe8d5

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Do not let a zero-amount lot posting declare a price

The acquisition test matched Positive, which includes a zero amount, so a
posting of no shares at all counted as a trade and its rate went into the price
graph.  With a later timestamp it then won over the rate a real purchase had
set: lot-zero.cent valued one share at 475.00 CHF instead of 95.00.

Also corrects the combineLots property that let this class of thing through in
its own right.  MultiAccount drops a key whose balance reaches zero, so the
result is not keyed by every underlying currency of the input, and random
amounts never cancel exactly, so QuickCheck agreed.  Stated per currency now,
with a fixed case for the cancellation.

Timeline