pretty: support "mboxrd" output format

This output format prevents format-patch output from breaking
readers if somebody copy+pasted an mbox into a commit message.

Unlike the traditional "mboxo" format, "mboxrd" is designed to
be fully-reversible.  "mboxrd" also gracefully degrades to
showing extra ">" in existing "mboxo" readers.

This degradation is preferable to breaking message splitting
completely, a problem I've seen in "mboxcl" due to having
multiple, non-existent, or inaccurate Content-Length headers.

"mboxcl2" is a non-starter since it's inherits the problems
of "mboxcl" while being completely incompatible with existing
tooling based around mailsplit.

ref: http://homepage.ntlworld.com/jonathan.deboynepollard/FGA/mail-mbox-formats.html

Signed-off-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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