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).
