If the source encoding is not UTF-8 we need to encode the data as
`JSON.dump` may throw an error if the input can not be converted to
UTF-8. We only encode when necessary to reduce the overhead.
Fixesgitlab-org/gitlab-ce#25489
By passing commit data to this worker we remove the need for querying
the Git repository for every job. This in turn reduces the time spent
processing each job.
The migration included migrates jobs from the old format to the new
format. For this to work properly it requires downtime as otherwise
workers may start producing errors until they're using a newer version
of the worker code.
Robert Speicher