handle dir

-- missingrepo-git/index.html --
<!DOCTYPE html>
<html>
<meta name="go-import" content="vcs-test.golang.org/go/missingrepo/missingrepo-git git https://vcs-test.golang.org/git/missingrepo">
-- missingrepo-git/notmissing/index.html --
<!DOCTYPE html>
<html>
<meta name="go-import" content="vcs-test.golang.org/go/missingrepo/missingrepo-git/notmissing git https://vcs-test.golang.org/git/mainonly">
