imap-send.c: set CURLOPT_USE_SSL to CURLUSESSL_TRY

According to the cURL documentation for the CURLOPT_USE_SSL option,
it is only used with plain text protocols that get upgraded to SSL
using the STARTTLS command.

The server.use_ssl variable is only set when we are using a protocol
that is already SSL/TLS (i.e. imaps), so setting CURLOPT_USE_SSL
when the server.use_ssl variable is set has no effect whatsoever.

Instead, set CURLOPT_USE_SSL to CURLUSESSL_TRY when the server.use_ssl
variable is NOT set so that cURL will attempt to upgrade the plain
text connection to SSL/TLS using STARTTLS in that case.

This much more closely matches the behavior of the non-cURL code path.
Signed-off-by: NKyle J. McKay <mackyle@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

imap-send.c

@@ -1421,8 +1421,8 @@ static CURL *setup_curl(struct imap_server_conf *srvc)
 		strbuf_release(&auth);
 	}
 
-	if (server.use_ssl)
-		curl_easy_setopt(curl, CURLOPT_USE_SSL, (long)CURLUSESSL_ALL);
+	if (!server.use_ssl)
+		curl_easy_setopt(curl, CURLOPT_USE_SSL, (long)CURLUSESSL_TRY);
 
 	curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, server.ssl_verify);
 	curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, server.ssl_verify);