Add submodule links in tree listing

When a submodule occurs in a tree, generate a link to show the
module/commit. The link is specified as a sprintf string in /etc/cgitrc,
using parameters 'module-link' and 'repo.module-link'. This should probably
be extended with repo.module-link.$path.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>