When the prefix is used (e.g. not /), the links generated in the
index page are wrong. In this patch the relative path is correctly
calculated by the pathlib module and a absolute url is used as link
in the index page.

The original bug can be demonstrated with following example:

---
import pathlib

from aiohttp import web

app = web.Application()
app.router.add_static('/static', pathlib.Path(__file__).parent,
show_index=True)

web.run_app(app)

---