commit: reject non-characters

Unicode clause D14 defines all characters U+nFFFE and U+nFFFF (where
0 <= n <= 10h) as well as the range U+FDD0..U+FDEF as non-characters,
reserved for internal use only.  Disallow these characters in commit
messages as they are normally not recommended for interchange.

Signed-off-by: Peter Krefting <peter@softwolves.pp.se>
Signed-off-by: Junio C Hamano <gitster@pobox.com>