It appears that a lot of folks have gone to replacing git clone with the combo git init && git fetch. This seems rather silly, and unfortunately tools like Jenkins won't do that for you. So why does git clone not take a refspec, the same way git fetch does?
Specifically, if you wish to run a gerrit triggered build task on Jenkins, you need to ensure that the workspace exists, otherwise jenkins will fail to checkout the revision containing the gerrit change. This is because gerrit uses a ref path that is not among those fetched by a git clone.