orig tarball absolute symlink breaks moving the checkout to another system

[Andreas Hasenack]

The orig tarball symlink that git-ubuntu creates as a result of export-orig is absolute:

```
drwxrwxr-x 29 andreas andreas 50 jan 5 10:40 unbound
lrwxrwxrwx 1 andreas andreas 113 jan 5 10:41 unbound_1.13.1.orig.tar.gz -> /home/andreas/git/packages/unbound/unbound/.git/git-ubuntu-cache/ubuntu/unbound/1.13.1/unbound_1.13.1.orig.tar.gz
```

In my workflow, I sometimes rsync that directory structure to a container, to test builds, and the symlink is then broken because the container does not have an `andreas` user, just `ubuntu`.

My suggestion is to make that symlink relative:
```
drwxrwxr-x 29 andreas andreas 50 jan 5 10:40 unbound
lrwxrwxrwx 1 andreas andreas 113 jan 5 10:41 unbound_1.13.1.orig.tar.gz -> unbound/.git/git-ubuntu-cache/ubuntu/unbound/1.13.1/unbound_1.13.1.orig.tar.gz
```