mailinfo: allow -u to fall back on latin1 to utf8 conversion.

mailinfo: allow -u to fall back on latin1 to utf8 conversion.

When the message body does not identify what encoding it is in,
-u assumes it is in latin-1 and converts it to utf8, which is
the recommended encoding for git commit log messages.

With -u=<encoding>, the conversion is made into the specified
one, instead of utf8, to allow project-local policies.

Signed-off-by: Junio C Hamano <junkio@cox.net>