We only know the tag SHA after we create the tag.
This means that we pass a different value to the hooks that happen before
creating the tag, and a different value to the hooks that happen after
creating the tag.
This is not an ideal situation, but it is a trade-off we decided to
make. For discussion of the alternatives please refer to
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7700#note_18982873
"pre-receive" and "update" hooks always get the SHA of the commit
that the tag points to. "post-receive" gets the tag SHA if it is
an annotated tag or the commit SHA if it is an lightweight tag.
Currently we always create annotated tags if UI is used.
Lin Jen-Shin