http-backend: fix die recursion with custom handler

http-backend: fix die recursion with custom handler

When we die() in http-backend, we call a custom handler that
writes an HTTP 500 response to stdout, then reports the
error to stderr. Our routines for writing out the HTTP
response may themselves die, leading to us entering die()
again.

When it was originally written, that was OK; our custom
handler keeps a variable to notice this and does not
recurse. However, since cd163d4 (usage.c: detect recursion
in die routines and bail out immediately, 2012-11-14), the
main die() implementation detects recursion before we even
get to our custom handler, and bails without printing
anything useful.

We can handle this case by doing two things:

  1. Installing a custom die_is_recursing handler that
     allows us to enter up to one level of recursion. Only
     the first call to our custom handler will try to write
     out the error response. So if we die again, that is OK.
     If we end up dying more than that, it is a sign that we
     are in an infinite recursion.

  2. Reporting the error to stderr before trying to write
     out the HTTP response. In the current code, if we do
     die() trying to write out the response, we'll exit
     immediately from this second die(), and never get a
     chance to output the original error (which is almost
     certainly the more interesting one; the second die is
     just going to be along the lines of "I tried to write
     to stdout but it was closed").

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>