Log check result status in mergeability logger

What does this MR do and why?

Prior to this change, mergeability logger only logged the output of the #success? method. This is not granular enough:

1. The status can be :inactive, which is neither successful nor unsuccessful. So can be false negatives in there.
2. The combined actually calls #unsuccessful? and succeeds if no check is unsuccessful. The #unsuccessful? method is implemented as failed? || checking?. So if the check fails, we don't know if it's intended to be temporary (#checking?) a real failure (#failed?).

We recently ran into this debugging a failed auto merge, where it the approval check appears to have failed, but we can't pin down whether it was due to value from the PostgreSQL or Redis.

With this change, we also log the actual status, so that we can differentiate between different types of successes and failures.

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