hgweb: use native strings when interfacing with stdlib headers
We're still parsing the stdlib-provided headers here, so we need to
tread carefully and use native strings. Yuck.
Differential Revision: https://phab.mercurial-scm.org/D3250
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}