Merge branch 'jk/http-backend-deadlock'

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