zoekt-mirror-gerrit: fix fetch for meta/config (#777)
The current configuration creates a conflict with default
fetch config `+refs/heads/*:refs/heads/*`.
The first fetch is ok but once a local "refs/heads/meta/config" exists
fetch fails because 2 fetch rules match for local "refs/heads/meta/config" :
"refs/meta/config" and "refs/heads/meta/config"
Changing local ref name fixes the problem