Fetch pipelines by merge request instead of branch

[JaimeLennox]

... if possible.

Since we operate on merge requests this makes more sense, but the feature was only available from version 10.5.

It allows us to fetch all pipelines for a particular merge request, including detached ones, which would otherwise not be fetched due to a different ref format.

An annoyance of this route is that it doesn't allow ordering or sorting, which we use to determine the latest pipeline by. However it's very simple to just do this ourselves; the only downside is this is no longer
customizable, but nothing in the codebase appears to use this feature anyway.

The batch job tries cancel pipelines by their status, which we can't filter by for this route (unless we do it manually as well) so I've left that using the old method. This can mean some detached pipelines may
continue running after a batch job has completed, but this seems like a very minor issue.

Fixes: #185

[b1rdex]

This causes an error if ci is configured to run different tasks both on the branch and the merge request.

I have tests and all other things configured to be run on the branches because such tasks don't need a merge request to be run. Also, I have one task configured to be run on merge requests — it is used to toggle merge request settings (enable squash). So now Marge thinks that this simple task is my tests and merges even if actual tests are running and not finished yet.

Looks like that's because current_pipeline = next(iter(pipeline for pipeline in pipelines if pipeline.sha == commit_sha), None) line — Marge takes just one pipeline in accordance.

[b1rdex]

Could we please make this behavior switchable through the config file?

[b1rdex]

Extracted issue from comments to #227.