Concurrent wiki updates cause git exceptions

It appears that a user has scripted wiki updates in nodejs. This likely sent concurrent requests, and they were not serialized correctly.

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1644213/?referrer=gitlab_plugin

GRPC::Unknown: 2:Gitlab::Git::CommitError: Could not update master. Please refresh and try again.
  grpc/generic/active_call.rb:31:in `check_status'
    fail GRPC::BadStatus.new_status_exception(
  grpc/generic/active_call.rb:181:in `attach_status_results_and_complete_call'
    recv_status_batch_result.check_status
  grpc/generic/active_call.rb:418:in `client_streamer'
    attach_status_results_and_complete_call(batch_result)
  grpc/generic/client_stub.rb:255:in `block in client_streamer'
    c.client_streamer(requests, metadata: metadata)
  grpc/generic/interceptors.rb:170:in `intercept!'
    return yield if @interceptors.none?
...
(157 additional frame(s) were not displayed)

Gitlab::Git::CommandError: 2:Gitlab::Git::CommitError: Could not update master. Please refresh and try again.
  gitlab/git/wraps_gitaly_errors.rb:13:in `rescue in wrapped_gitaly_errors'
    raise Gitlab::Git::CommandError.new(e)
  gitlab/git/wraps_gitaly_errors.rb:6:in `wrapped_gitaly_errors'
    def wrapped_gitaly_errors(&block)
  gitlab/git/wiki.rb:84:in `update_page'
    wrapped_gitaly_errors do
  wiki.rb:145:in `update_page'
    wiki.update_page(page.path, title || page.name, format.to_sym, content, commit)
  wiki_page.rb:240:in `block in update'
    wiki.update_page(
...
(144 additional frame(s) were not displayed)

Gitlab::Git::CommandError: 2:Gitlab::Git::CommitError: Could not update master. Please refresh and try again.