Merge branch 'jk/http-backend-deadlock'

Communication between the HTTP server and http_backend process can lead to a dead-lock when relaying a large ref negotiation request. Diagnose the situation better, and mitigate it by reading such a request first into core (to a reasonable limit). * jk/http-backend-deadlock: http-backend: spool ref negotiation requests to buffer t5551: factor out tag creation http-backend: fix die recursion with custom handler