Use ``strings.TrimPrefix()`` for relative paths instead of
``strings.TrimLeft()``, since ``TrimLeft`` takes in a cutset instead of
a prefix sub-string.

In this particular case, if the ``--root`` (output) directory is
``/tmp/foo`` and we are determining the relative path for
``/tmp/foo/tmp-foo``, the relative path would result in ``/-foo`` since
``/tmp/foo/tmp`` would be matched by the ``/tmp/foo/`` cutset.

This results in a flawed map of ``knownPaths`` inside slicer.go, which
is only checked by the mutation script functions such as
``content.list()``.

This commit fixes said bug.