[gitbug] Stop fetching closed issues

I have been an on-and-off user of bugwarrior with the git-bug service for a few years now and one thing that has always stopped me is that it fetches _all_ issues from the issue tracker (open and closed) into taskwarrior.

I now understand that the other services do not do this, and this may be a bug in the implementation?

Current behavior:

- Context: Repository with git-bug issue tracker with one open and one closed issue.
- `bugwarrior pull` imports _both_ issues into taskwarrior as pending tasks.
- I close the 2nd issue in git-bug.
- `bugwarrior pull` keeps both issues imported as pending tasks in taskwarrior.

Manually completing in task (`task done`) and pulling will mark them as `status:pending` again in taskwarrior even if they are closed in git-bug (due to bugwarrior's pull-only model).
Only issues that are **deleted** in git-bug with `git bug bug rm ` will be marked completed in taskwarrior.

I believe this goes against the intended UX in the issue tracker (i.e. close finished issues instead of deleting them).

Expected behavior:

- Context: Repository with git-bug issue tracker with one open and one closed issue.
- `bugwarrior pull` imports only the open issue into taskwarrior as pending.
- I close the 2nd issue in git-bug.
- `bugwarrior pull` marks the 2nd issue's task as completed.

Taking a look at other Services, e.g. [GitHub](https://github.com/GothenburgBitFactory/bugwarrior/blob/4ed78ac2734e0979268d41ff22ef8b6d4a7a40ee/bugwarrior/services/github.py#L441) restricts queries to `status:open`, as does [GitLab](https://github.com/GothenburgBitFactory/bugwarrior/blob/4ed78ac2734e0979268d41ff22ef8b6d4a7a40ee/bugwarrior/services/gitlab.py#L325).

My question: is the current behavior the intended behavior for bugwarrior?

For the moment I am using a simple [fix which drops](https://github.com/GothenburgBitFactory/bugwarrior/commit/2a187e7032507bab82686fe6e87dc19f5e444605) closed issues before importing.
If this is indeed a bug and not intended I am happy to prepare a more polished version as PR for bugwarrior.

Start with the git-bug service used by `bugwarrior pull`, then compare its issue filtering with the referenced GitHub and GitLab service implementations in `github.py` and `gitlab.py`. Reproduce the open-and-closed issue scenario and verify that closed issues are excluded from import and existing tasks become completed after pulling.

Written by the indexing model from the issue text.