Always compare with FETCH_HEAD in downtime_check
Always compare with FETCH_HEAD in downtime_check

This ensures this CI step works properly even when doing a shallow
clone.

LUMC/gitlabhq,darkrasid/gitlabhq,SVArago/gitlabhq,jirutka/gitlabhq,stoplightio/gitlabhq,iiet/iiet-git,jirutka/gitlabhq,openwide-java/gitlabhq,mr-dxdy/gitlabhq,allysonbarros/gitlabhq,icedwater/gitlabhq,jirutka/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,htve/GitlabForChinese,dreampet/gitlab,openwide-java/gitlabhq,mr-dxdy/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,allysonbarros/gitlabhq,dplarson/gitlabhq,t-zuehlsdorff/gitlabhq,SVArago/gitlabhq,htve/GitlabForChinese,shinexiao/gitlabhq,dplarson/gitlabhq,t-zuehlsdorff/gitlabhq,openwide-java/gitlabhq,SVArago/gitlabhq,axilleas/gitlabhq,mr-dxdy/gitlabhq,axilleas/gitlabhq,daiyu/gitlab-zh,allysonbarros/gitlabhq,screenpages/gitlabhq,screenpages/gitlabhq,shinexiao/gitlabhq,icedwater/gitlabhq,dreampet/gitlab,mmkassem/gitlabhq,daiyu/gitlab-zh,darkrasid/gitlabhq,iiet/iiet-git,shinexiao/gitlabhq,daiyu/gitlab-zh,daiyu/gitlab-zh,icedwater/gitlabhq,mmkassem/gitlabhq,dreampet/gitlab,axilleas/gitlabhq,mmkassem/gitlabhq,darkrasid/gitlabhq,t-zuehlsdorff/gitlabhq,openwide-java/gitlabhq,t-zuehlsdorff/gitlabhq,SVArago/gitlabhq,screenpages/gitlabhq,dplarson/gitlabhq,stoplightio/gitlabhq,LUMC/gitlabhq,htve/GitlabForChinese,iiet/iiet-git,dreampet/gitlab,stoplightio/gitlabhq,LUMC/gitlabhq,iiet/iiet-git,shinexiao/gitlabhq,darkrasid/gitlabhq,allysonbarros/gitlabhq,screenpages/gitlabhq,dplarson/gitlabhq,icedwater/gitlabhq,htve/GitlabForChinese,LUMC/gitlabhq,mr-dxdy/gitlabhq