`--flavour` marks as completed all tasks of non-selected targets

Hi there,

I've got something like the following configuration:

```
[general]

targets = test_jira

[flavor.test]
targets = test_jira

[flavor.prod]
targets = prod_jira

[test_jira]
service = jira
jira.base_uri = ...

[prod_jira]
service = jira

jira.base_uri = ...
...

[notifications]
notifications = True
backend = gobject
# only_on_new_tasks = True
```

Before running this on my intended production jira server, I wanted to verify that it worked as expected on a test jira server. I did that using `bugwarrior-pull --flavour test` and it worked fine.

Then I wanted to run it on the `prod` flavour. When I executed `bugwarrior-pull --flavour prod` it indeed fetched all the tasks from the prod jira server as expected, however at the end of the run it marked all the tasks of the `test` flavour as completed, which is not the behavior I would expect.