Refactor merge service logging again

This commit moves some the common logic for logging information and error messages into MergeRequests::BaseService and standardizes the fields used in the log message. This was needed because log_error was implemented in both MergeRequests::BaseService and accidentally overridden in MergeRequests::MergeService in !100381 (merged).

In addition, this moves a merge error message from application_json.log to git_json.log to be consistent with the other merge services.

This is done in preparation of adding a retry for critical database updates.

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