A `repo_model.Mirror` repository field (`.Repo`) will not automatically
be set, but is used without checking in mirror_pull.go:UpdateAddress.
This will cause an NPE.
This PR changes UpdateAddress to use the helper function GetRepository()
helping prevent future NPEs but also changes modules/context/repo.go to
ensure that the Mirror.Repo is set.
Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
e32ab429ff