send-email: auth plain/login fix

send-email: auth plain/login fix

git send-email was not authenticating properly when communicating over
TLS with a server supporting only AUTH PLAIN and AUTH LOGIN. This is
e.g. the standard server setup under debian with exim4 and probably
everywhere where system accounts are used.

The problem (only?) exists when libauthen-sasl-cyrus-perl
(Authen::SASL::Cyrus) is installed. Importing Authen::SASL::Perl
makes Authen::SASL use the perl implementation which works
better.

The solution is based on this forum thread:
http://www.perlmonks.org/?node_id=904354.

This patch is tested by sending it. Without this fix, the interaction with
the server failed like this:

$ git send-email --smtp-encryption=tls --smtp-server=... --smtp-debug=1 change1.patch
...
Net::SMTP::SSL=GLOB(0x238f668)<<< 250-AUTH LOGIN PLAIN
Password:
Net::SMTP::SSL=GLOB(0x238f668)>>> AUTH
Net::SMTP::SSL=GLOB(0x238f668)<<< 501 5.5.2 AUTH mechanism must be specified
5.5.2 AUTH mechanism must be specified

Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>