When using a mailing list for the patch review process, threading is
important.  Threading is done not only by subject, but also (in most
mailreaders, at least) by the In-Reply-To and References headers.

It's quite common for a patch to be rejected with a "please amend with
<trivial change> and resend".

After making the amendment, the user has two options: use "darcs send"
and break the threading, or use "darcs send -o" and have to manually
attach the patch bundle to a manual reply, manually setting the
subject and message body and the MIME type of the patch bundle.

It would be useful if the user could simply copy the "please amend"
message's Message-ID onto the command line and run something like

    darcs send --in-reply-to '<fab7afd@foo.org>'

Which would be the same as a normal "darcs send", but include an
In-Reply-To.  This would make threading work better.

Of course, it will probably become useful to add arbitrary headers in
future.  For example, within a company one might have a single patch
review list, and distinguish between different patch repositories by
adding this to _darcs/prefs/prefs in each repo:

   send header X-Darcs-Patch-For: /srv/darcs/libsnotty
   send header X-Bluebottle: Thinks: I have half a pound of dolly mixture

Note that with arbitrary --header it'll be necessary to support
multiple --header arguments.

Resolved by

Mon Nov 17 10:20:56 GMT 2008  Pavel Shramov <shramov@mexmat.net>
  * Add --in-reply-to flag for send command