I wrote this stuff a while ago and forgot to write tests.  Wanted
to do so now to wrap up the PR and immediately found problems.

The base for the relative urls is determined as follows, with descending
priority:

- remote url of HEAD's remote tracking branch
- remote "origin"
- workdir

This follows git.git behaviour

`git_submodule` objects were already refcounted internally in case
the submodule name was different from the path at which it was
stored.  This makes that refcounting externally used as well, so
`git_submodule_lookup` and `git_submodule_add_setup` return an
object that requires a `git_submodule_free` when done.

This removes the fetchRecurse compiler warnings and makes the
behavior match the other submodule options (i.e. the in-memory
setting can be reset to the on-disk value).