send-email: make --no-chain-reply-to the default

send-email: make --no-chain-reply-to the default

In http://article.gmane.org/gmane.comp.version-control.git/109790 I
threatened to announce a change to the default threading style used by
send-email to no-chain-reply-to (i.e. the second and subsequent messages
will all be replies to the first one), unless nobody objected, in 1.6.3.

Nobody objected, as far as I can dig the list archive.  But when nothing
happened in 1.6.3 nor 1.6.4, nobody from the camp who complained loudly
that led to the message did not complain either.

So I am guessing that after all nobody cares about this.  But 1.7.0 is a
good time to change this, and as I said in the message, I personally think
it is a good change, so here it is.

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