www: fix absolute URLs when mounted under a subdir
While we avoid generating absolute URLs in most cases, our
"git clone" instructions and URL headers in mboxrd files
contain full URLs.
So do the same thing we do for WwwAtomStream and pre-generate
the full URL before Plack::App::URLMap changes $env->{PATH_INFO}
and $env->{SCRIPT_NAME} back to their original values.
Reported-by: edef <edef@edef.eu>
Link: https://public-inbox.org/meta/cover.0f97c47bb88db8b875be7497289d8fedd3b11991.1569296942.git-series.edef@edef.eu/