Log check result status in mergeability logger

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.

Changelog: changed