NoMethodError: undefined method `>' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/673322/

NoMethodError: undefined method `>' for nil:NilClass
  merge_request.rb:1306:in `has_commits?'
    merge_request_diff && commits_count > 0
  gitlab/metrics/instrumentation.rb:161:in `block in has_commits?'
    .measure { super }
  gitlab/metrics/method_call.rb:36:in `measure'
    retval = yield
  gitlab/metrics/instrumentation.rb:161:in `has_commits?'
    .measure { super }
  merge_request.rb:1310:in `has_no_commits?'
    !has_commits?
...
(68 additional frame(s) were not displayed)

undefined method `>' for nil:NilClass