t0060: add tests for prefix_path when path begins with work tree

t0060: add tests for prefix_path when path begins with work tree

One edge-case that isn't currently checked in the tests is the beginning
of the path matching the work tree, despite the target not actually
being the work tree, for example:

  path = /dir/repoa
  work_tree = /dir/repo

should fail since the path is outside the repo. However, if /dir/repoa
is in fact a symlink that points to /dir/repo, it should instead
succeed.

Add two tests covering these cases, since they might be potential
regression points.

Signed-off-by: Martin Erik Werner <martinerikwerner@gmail.com>
Reviewed-by: Duy Nguyen <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>