send-email: provide whitelist of SMTP AUTH mechanisms

When sending an e-mail, the client and server must agree on an
authentication mechanism. Some servers (due to misconfiguration
or a bug) deny valid credentials for certain mechanisms. In this
patch, a new option --smtp-auth and configuration entry smtpAuth
are introduced. If smtp_auth is defined, it works as a whitelist
of allowed mechanisms for authentication selected from the ones
supported by the installed SASL perl library.

Signed-off-by: Jan Viktorin <viktorin@rehivetech.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-send-email.txt b/Documentation/git-send-email.txt
index 7ae467b..631f6c8 100644
--- a/Documentation/git-send-email.txt
+++ b/Documentation/git-send-email.txt
@@ -171,6 +171,19 @@
 	to determine your FQDN automatically.  Default is the value of
 	'sendemail.smtpDomain'.
 
+--smtp-auth=<mechanisms>::
+	Whitespace-separated list of allowed SMTP-AUTH mechanisms. This setting
+	forces using only the listed mechanisms. Example:
++
+------
+$ git send-email --smtp-auth="PLAIN LOGIN GSSAPI" ...
+------
++
+If at least one of the specified mechanisms matches the ones advertised by the
+SMTP server and if it is supported by the utilized SASL library, the mechanism
+is used for authentication. If neither 'sendemail.smtpAuth' nor '--smtp-auth'
+is specified, all mechanisms supported by the SASL library can be used.
+
 --smtp-pass[=<password>]::
 	Password for SMTP-AUTH. The argument is optional: If no
 	argument is specified, then the empty string is used as