Don't depend on 'git fetch --all' existing

Change-Id: I6172e27e0f5c32952db6edc2a71c6ba681060dd9
Reviewed-on: http://review.couchbase.org/41961
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>