Error 500 viewing diffs.json

Can be seen in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14543/diffs.json.

https://sentry.gitlab.net/gitlab/gitlabcom/issues/589058/

NoMethodError: undefined method `[]' for nil:NilClass
  commit.rb:79:in `truncate_sha'
    sha[0..MIN_SHA_LENGTH]
  git_helper.rb:9:in `short_sha'
    Commit.truncate_sha(text)
  merge_request_diff_entity.rb:24:in `block in <class:MergeRequestDiffEntity>'
    short_sha(merge_request_diff.head_commit_sha)
  grape_entity/entity.rb:494:in `instance_exec'
    instance_exec(object, &block)
  grape_entity/entity.rb:494:in `exec_with_object'
    instance_exec(object, &block)
...
(172 additional frame(s) were not displayed)

NoMethodError: undefined method `[]' for nil:NilClass