format-patch -s: add MIME encoding header if signer's name requires so

format-patch -s: add MIME encoding header if signer's name requires so

When the body of the commit log message contains a non-ASCII character,
format-patch correctly emitted the encoding header to mark the resulting
message as such.  However, if the original message was fully ASCII, the
command line switch "-s" was given to add a new sign-off, and
the signer's name was not ASCII only, the resulting message would have
contained non-ASCII character but was not marked as such.

This was cherry-picked from the fix in 'master'

Signed-off-by: Junio C Hamano <gitster@pobox.com>