Merge branch 'lh/send-email-hide-x-mailer'

"git send-email" normally identifies itself via X-Mailer: header in the message it sends out. A new command line flag allows the user to squelch the header. * lh/send-email-hide-x-mailer: test/send-email: --[no-]xmailer tests send-email: add --[no-]xmailer option