imap-send.c: introduce the GIT_TRACE_CURL enviroment variable

Permit the use of the GIT_TRACE_CURL environment variable calling
the setup_curl_trace http.c helper routine.

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>

imap-send.c

diff --git a/imap-send.c b/imap-send.c
index 2c52027..8ad8067 100644
--- a/imap-send.c
+++ b/imap-send.c
@@ -1443,6 +1443,7 @@
 
 	if (0 < verbosity || getenv("GIT_CURL_VERBOSE"))
 		curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
+	setup_curl_trace(curl);
 
 	return curl;
 }