Fails when `walk`ing an absolute symlinked path with a `ROOT_MARKER`

[zsol]

This happens when you fetch a project with a ROOT_MARKER (i.e. .git, .hg, pyproject.toml) into MacOS's system temp.

Easy repro:

tmp=$(mktemp -d)
ln -s "$tmp" foo
touch foo/.git
python -c 'import trailrunner; from pathlib import Path; next(trailrunner.walk(Path.cwd() / "foo"))'

BOOM:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "xxxxlib/python3.11/site-packages/trailrunner/core.py", line 169, in gen
    relative = child.relative_to(root)
               ^^^^^^^^^^^^^^^^^^^^^^^
  File "xxxxlib/python3.11/pathlib.py", line 730, in relative_to
    raise ValueError("{!r} is not in the subpath of {!r}"
ValueError: 'xxxx/foo' is not in the subpath of '/tmp' OR one path is relative and the other is absolute.