Git allows two kinds of tags: annotated and plain. Plain tags are
simple refs pointing to a commit, and annotated tags point to a Tag
object then pointing to the commit. The code previously assumed that
all exported tags where plain tags, which would overwrite annotated
tags.

We recently fixed this, in cc4f3340afc9, when exporting tags from
Mercurial to Git; this applies the same fix during Git discovery.

Fixes #358