remote: return problem refspecs instead of using a callback
There is no reason why we need to use a callback here. A string array
fits better with the usage, as this is not an event and we don't need
anything from the user.
git_submodule_resolve_url supports relative urls
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