cgi: use transaction_complete when stdout goes away

transaction_complete includes processing for the case we are holding
an output-side buflist, and arranges for it to be sent before the
connection is actually closed.

This stops very slow links not being ablet to use git clone via
cgi mount, because the close races the flushing of the output-side
buflist and in the case the connection is bad, closes before
everything was sent.