CI status bug

[vermiculus]

trafficstars

• When the push-remote is configured but the branch by the push-target branch doesn't exist yet:

  error("Invalid branch name %s" "tarsius/master")
  magit-split-branch-name("tarsius/master")
  magithub-ci-status--get-default-ref()
  magithub-insert-ci-status-header()
  magithub-maybe-insert-ci-status-header()

From #175