format.numbered is a tri-state variable.  Boolean values enable or
disable numbering by default and "auto" enables number when outputting
more than one patch.

--no-numbered (short: -N) will disable numbering.
Signed-off-by: NBrian Gernhardt <benji@silverinsanity.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>