Gitiles instance returns voluntarily a malformed json output (json prefixed with
``)]}'\n``) [2]. The lister deals with it to properly parse the json response
nonetheless. It drops the prefix and then parses the json.

If at some point, they drop this prefix to return json directly, the lister will be able
to deal with it too. There are 2 tests one with 'standard' gitile format and another
with standard json to account for both case.

Refs. swh/meta#5045

[2] https://github.com/google/gitiles/issues/263