http.c: implement the GIT_TRACE_CURL environment variable
Implement the GIT_TRACE_CURL environment variable to allow a
greater degree of detail of GIT_CURL_VERBOSE, in particular
the complete transport header and all the data payload exchanged.
It might be useful if a particular situation could require a more
thorough debugging analysis. Document the new GIT_TRACE_CURL
environment variable.
Helped-by: Torsten Bögershausen <tboegi@web.de> Helped-by: Ramsay Jones <ramsay@ramsayjones.plus.com> Helped-by: Junio C Hamano <gitster@pobox.com> Helped-by: Eric Sunshine <sunshine@sunshineco.com> Helped-by: Jeff King <peff@peff.net> Signed-off-by: Elia Pinto <gitter.spiros@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>