Merge branch 'ga/send-email-sendmail-cmd'

"git send-email" learned the "--sendmail-cmd" command line option and the "sendemail.sendmailCmd" configuration variable, which is a more sensible approach than the current way of repurposing the "smtp-server" that is meant to name the server to instead name the command to talk to the server. * ga/send-email-sendmail-cmd: git-send-email: add option to specify sendmail command