http.c: implement the GIT_TRACE_CURL environment variable

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 <redacted>
Helped-by: Ramsay Jones <redacted>
Helped-by: Junio C Hamano <redacted>
Helped-by: Eric Sunshine <redacted>
Helped-by: Jeff King <redacted>
Signed-off-by: Elia Pinto <redacted>
Signed-off-by: Junio C Hamano <redacted>