Rugged::ReferenceError: Revspec '0000000000000000000000000000000000000000' not found.

https://sentry.gitlap.com/gitlab/gitlabcom/issues/2235/

Rugged::ReferenceError: Revspec '0000000000000000000000000000000000000000' not found.
  lib/gitlab/metrics/instrumentation.rb:138:in `rev_parse'
    retval   = #{send_signature}
  lib/gitlab/metrics/instrumentation.rb:138:in `diff'
    retval   = #{send_signature}
  app/services/git_push_service.rb:68:in `index_commits_blobs'
    @project.repository.index_blobs(from_rev: params[:oldrev], to_rev: params[:newrev])
  app/services/git_push_service.rb:60:in `update_merge_requests'
    index_commits_blobs if Gitlab.config.elasticsearch.enabled
  app/services/git_push_service.rb:47:in `execute'
    update_merge_requests
...
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