git_tag: Find time after we get full ref

git show has different behaviour when passing tags instead of commits, causing our git show -s --format=%ct %{ref} call to be prefixed with the tag, tagger, tag message and pgp signature, and ruining the sorting by time. This ensures we are passing a precise commit to git show rather than just a tag name, so we get the correct output.

Fixes #33 (closed)

Branch name is poorly chosen as I thought the issue was caused by something else when I created it.