Provide explicit html format when rendering git_not_found page
Provide explicit html format when rendering git_not_found page

Prior, if the request format was, for example, .zip, we'd get an
`ActionView::MissingTemplate` error.

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