Remote objects are not meant to be changed from under the user. We did
this in rename, but only the name and left the refspecs, such that a
save would save the wrong refspecs (and a fetch and anything else would
use the wrong refspecs).

Instead, let's simply take a name and not change any loaded remote from
under the user.

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