Issue #268: 'pushtag' does not work with include (blais/beancount)

[Carbo Kuo]

New issue 268: 'pushtag' does not work with include
https://bitbucket.org/blais/beancount/issues/268/pushtag-does-not-work-with-include

Carbo Kuo:

For example:

```
#!yaml

pushtag #tag1
include 'another_file'
poptag #tag1
```

Expected behavior: all transactions in 'another_file' are tagged with #tag1

Actual behavior: no transaction is in 'another_file' tagged