Add a .gitreview file

Since subunit2sql was adopted by openstack-infra code submissions and
reviews will be done in openstack gerrit. This commit adds a
.gitreview file to subunit2sql, so that by default the git review
command will point to the right location for subunit2sql.

Change-Id: I043cbd59fb7c3e83de2dfe8922b0ab96c56cd37f