Fetch tag refs from zuul in gerrit-git-prep

* gerrit-git-prep.sh: If the zuul ref is a tag, then fetch the tag
from zuul. Otherwise there's a risk that it may not have replicated
to the origin yet when initially setting up the workspace.

Change-Id: Iadd8385e4a0d0957fcac1cf89d0b5a1cfa848b49
Merge "Fetch tag refs from zuul in gerrit-git-prep"