Merge branch 'jv/send-email-selective-smtp-auth'

"git send-email" learned a new option --smtp-auth to limit the SMTP
AUTH mechanisms to be used to a subset of what the system library
supports.

* jv/send-email-selective-smtp-auth:
send-email: provide whitelist of SMTP AUTH mechanisms

diff --cc Documentation/git-send-email.txt
Simple merge

diff --cc git-send-email.perl
Simple merge