hgweb: Pass only filename instead of full path when downloading raw files.
Before this patch "filename=foo/bar" was sent and e.g. Firefox offered to save
the file as "foo-bar" instead of just "bar".
% Without -v
access log created - .hg/hgrc respected
% With -v
listening at http://localhost/
% With --prefix foo
listening at http://localhost/foo/
% With --prefix /foo
listening at http://localhost/foo/
% With --prefix foo/
listening at http://localhost/foo/
% With --prefix /foo/
listening at http://localhost/foo/