Don't fail on submodules

Don't fail on submodules

Submodules will get listed prefixed with a '@', using a specific CSS class for
styling. The href will be set to the url of the submodule.
Filesize will appear as 0 and filemode will not be printed to avoid an awkward
mode: "?---------".

In writefilestree, we don't return anymore if an entry can't be categorized as
an object, but rather, fail if we can't retrieve its name.