Pass use_no_ff_batches into merge options

[mathyoudawson]

#256 introduced an error where the use use_no_ff_batches option was not passed into the
options correctly. Merge options are passed into the batch job class,
and use_no_ff_batches was not included in the merge options.

Thus creating the error:
AttributeError: 'MergeJobOptions' object has no attribute 'use_no_ff_batches'

This PR adds use_no_ff_batches to the merge options.

[RobertKirk]

Do you know why the tests you wrote for #256 didn't catch this error? It would be good to cover that, so we can't have a regression.

[mathyoudawson]

The merge_batch method i introduced was tested, but the caller was not. I was surprised that other tests for the accept_mr method did not pick this up. Will have a go at adding some tests for this :)
It also looks like the happy path for accept_mr is untested.

[RobertKirk]

Great, thanks. It's possible the caller's test mocks the merge_batch method.

[mathyoudawson]

@RobertKirk Sorry this has taken me ages.
I've been struggling to wrap my head around how these mocks work.

Every time accept_mr is called during testing the exception Someone pushed to branch while we were trying to merge is raised.
This is due to actual_sha being bound to a mock object <MagicMock name='mock.rebase()' id='4656111568'>
https://github.com/smarkets/marge-bot/blob/master/marge/batch_job.py#L139

Python is not a language I know very well so I feel some of this is going over my head. Would appreciate some help or a push in the right direction

[RobertKirk]

I think looking at it again I'm happy to just merge it in. I agree the mocks are confusing.

[mathyoudawson]

@RobertKirk Cheers for merging this. For what it's worth the company I work for has been running the above code in batch mode for the last week with no issues.