Fix Projects::UpdateMirrorService to allow tags pointing to blob objects

After the release of https://gitlab.com/gitlab-org/gitlab_git/merge_requests/103 we changed the way we return the target of tag objects, now it's a Commit object if there is some and nil in any other case, like when a tag points to a Blob object.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/21040

See merge request !673