Teach git_repository_init_ext to use relative paths for the gitlink
to the work directory. This is used when creating a sub repository
where the sub repository resides in the parent repository's
.git directory.
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.
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
