Xgit.Repository.put_ref/2 doesn't update the ref log.

[scouten]

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Describe What Is Incomplete
The function Xgit.Repository.put_ref/2 doesn't maintain or update the ref log.

Definition of Done

• An option is added to Xgit.Repository.put_ref/2 to provide a ref log message.
• An option is added to Xgit.Repository.put_ref/2 to suppress writing to the ref log.
• When Xgit.Repository.put_ref/2 is called for an OnDisk repository, the ref log is updated and the output matches command-line git.

Bonus Points

• An interface is added to Xgit.Repository for accessing the ref log. It can read ref logs written by command-line git.