Issue #163: Position tracking regression (blais/beancount)

[Rob Smith]

New issue 163: Position tracking regression
https://bitbucket.org/blais/beancount/issues/163/position-tracking-regression

Rob Smith:

After loading my file with beancount-2.0b15, I noticed a few errors related to position tracking. My beancount file had worked fine when I used it with the latest version a few months ago.

I've created a minimal reproducible example. The problem goes away if I remove either:

1) The starting balance, or

2) The shares withheld for taxes.

```
2014-12-31 open Assets:MorganStanley:RSU
2014-12-31 open Equity:Opening-Balances
2014-12-31 open Expenses:Taxes:Y2015:Federal
2014-12-31 open Income:Employer:RSU

2014-12-31 * "Opening Balance"
Assets:MorganStanley:RSU 350 AAPL {95.23 USD}
Equity:Opening-Balances

2015-01-31 * "RSU Vesting"
Income:Employer:RSU -5000.00 USD
Assets:MorganStanley:RSU 50 AAPL {100.00 USD}
Assets:MorganStanley:RSU -5 AAPL {100.00 USD}
Expenses:Taxes:Y2015:Federal 500.00 USD
```