git-cache-proxy: Abandon fetch attempt if client disconnects

git-cache-proxy: Abandon fetch attempt if client disconnects

The git client sends the command and then waits for our response.  We
can tell if it has disconnected, because our stdin (the socket) will
be readable.

This is a good idea because otherwise if we have clients with their
own (formal or informal) timeouts, we can build up a backlog of
queued useless fetch attempts.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>