GitLab tags removal: compare only on the GitLab side

Previously, the detection of tags removal was based on back-converting to Mercurial the existing GitLab tags, which cannot work in case of escaped names.

Instead, we perform the detection entirely on the GitLab side. We can still have collisions, but there's not much that can be done about it (except that publication rights will be necessary to actually change a GitLab tag due to such a collision).

We also make it clear in the case the user does not have sufficient permissions that the reported tag name is the GitLab one, and can be slightly different from the Mercurial one.

Closes heptapod#464 (closed) (see there for the bad consequences)