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>

Documentation/git-send-email.txt

@@ -84,7 +84,7 @@ See the CONFIGURATION section for 'sendemail.multiedit'.
 --in-reply-to=<identifier>::
 	Specify the contents of the first In-Reply-To header.
 	Subsequent emails will refer to the previous email
-	instead of this if --chain-reply-to is set (the default)
+	instead of this if --chain-reply-to is set.
 	Only necessary if --compose is also set.  If --compose
 	is not set, this will be prompted for.
 
@@ -171,8 +171,8 @@ Automating
 	email sent.  If disabled with "--no-chain-reply-to", all emails after
 	the first will be sent as replies to the first email sent.  When using
 	this, it is recommended that the first file given be an overview of the
-	entire patch series. Default is the value of the 'sendemail.chainreplyto'
-	configuration value; if that is unspecified, default to --chain-reply-to.
+	entire patch series. Disabled by default, but the 'sendemail.chainreplyto'
+	configuration variable can be used to enable it.
 
 --identity=<identity>::
 	A configuration identity. When given, causes values in the

git-send-email.perl

@@ -71,7 +71,7 @@ sub usage {
     --suppress-cc           <str>  * author, self, sob, cc, cccmd, body, bodycc, all.
     --[no-]signed-off-by-cc        * Send to Signed-off-by: addresses. Default on.
     --[no-]suppress-from           * Send to self. Default off.
-    --[no-]chain-reply-to          * Chain In-Reply-To: fields. Default on.
+    --[no-]chain-reply-to          * Chain In-Reply-To: fields. Default off.
     --[no-]thread                  * Use In-Reply-To: field. Default on.
 
   Administering:
@@ -188,7 +188,7 @@ sub do_edit {
 
 my %config_bool_settings = (
     "thread" => [\$thread, 1],
-    "chainreplyto" => [\$chain_reply_to, 1],
+    "chainreplyto" => [\$chain_reply_to, undef],
     "suppressfrom" => [\$suppress_from, undef],
     "signedoffbycc" => [\$signed_off_by_cc, undef],
     "signedoffcc" => [\$signed_off_by_cc, undef],      # Deprecated