send-email: ask confirmation if given encoding name is very short

send-email: ask confirmation if given encoding name is very short

Sometimes people respond "y<ENTER>" (or "yes<ENTER>") when asked
this question:

    Which 8bit encoding should I declare [UTF-8]?

We already have a mechanism to avoid accepting a mistyped e-mail
address (we ask to confirm when the given address lacks "@" in it);
reuse it to trigger the same confirmation when given a very short
answer.  As a typical charset name is probably at least 4 chars or
longer (e.g. "UTF8" spelled without the dash, or "Big5"), this would
prevent such a mistake.

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