Use tag_exists? in GitAccess#protected_tag?
Use tag_exists? in GitAccess#protected_tag?

This removes the need for retrieving the entire list of tags just to
check if a specific one exists.

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