Certain filters can incorrectly disable loading of completed.data

The `Filter::pendingOnly` method attempts to implement a logic to determine whether it's safe to skip loading of the `completed.data` file.

However, in certain cases, the current implementation can get it wrong, for example:
```
$ task status.not:pending count
0
```

This result is incorrect, as there are non-pending tasks in the database, as evidenced by a different query:
```
$ task +COMPLETED count
1100
```