format-patch: support format.mboxrd with --stdout

format-patch: support format.mboxrd with --stdout

mboxrd is a more robust output format when used with --stdout
and needs more exposure.  Introducing this config knob lets
users choose the more robust format for all their --stdout
uses.

Relying on --pretty=mboxrd and including all of pretty-formats.txt
in the `git format-patch' documentation would likely be
confusing to users.  Furthermore, this setting is useful across
multiple invocations.  So introduce `format.mboxrd' as a boolean
configuration knob that changes the default --pretty=email format
to --pretty=mboxrd when (and only when) --stdout is in use.

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