t9001: send-email interation with --in-reply-to and --chain-reply-to

t9001: send-email interation with --in-reply-to and --chain-reply-to

 1. When --in-reply-to gives $reply_to, the first one becomes a reply to
    that message, with or without --chain-reply-to.

 2. When --chain-reply-to is in effect, all the messages are strung
    together to form a single chain.  The first message may be in reply to
    the $reply_to given by --in-reply-to command line option (see
    previous), or the root of the discussion thread.  The second one is a
    response to the first one, and the third one is a response to the
    second one, etc.

 3. When --chain-reply-to is not in effect:

    a. When --in-reply-to is used, too, the second and the subsequent ones
       become replies to $reply_to.  Together with the first rule, all
       messages become replies to $reply_to given by --in-reply-to.

    b. When --in-reply-to is not used, presumably the second and
       subsequent ones become replies to the first one, which would be the
       root.

The documentation is reasonably clear about the 1., 2. and 3a. above, I
think, even though I do not think 3b. is clearly specified.

The two tests added by this patch at least documents what happens between
these two options.

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