Fix Gerrit change ref string prefix

Created by: marekweb

Fix the format for the ref string for Gerrit changes, which causes a bug when the change ID is longer than two characters.

A change ref has the format refs/changes/X/Y/Z where X is the last two digits of the change number, Y is the entire change number, and Z is the patch set. For example, if the change number is 263270, the ref would be refs/changes/70/263270/2 for the second patch set.

Source: Gerrit documentation