Peek attempts to serialize results with `to_json`, which calls
`ActiveSupport::JSON`. If an object is passed to `to_json` that contains
instance variables, `ActiveSupport` will attempt to recursively traverse
all variables.

The problem is that we can get into an infinite loop if the instance
references to an instance that references to something else that points
back to the same instance.

To avoid this mess, we just call `to_s` on the object. It appears only
`Gitlab::Git::Repository` and `::Repository` are the culprits here.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/65404