t/lib-httpd: only route auth/dumb to dumb repos

t/lib-httpd: only route auth/dumb to dumb repos

Our test apache config points all of auth/ directly to the
on-disk repositories via an Alias directive. This works fine
because everything authenticated is currently in auth/dumb,
which is a subset.  However, this would conflict with a
ScriptAlias for auth/smart (which will come in future
patches), so let's narrow the Alias.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>