ez: introduce --in-reply-to for send

[alberand]

When sending numerous patchsets touching multiple projects, the structure of the email thread could be top-level cover letter with patchsets as subthreads (replies). As an example, while developing a filesystem feature one can also develop tests for xfstests for that feature.

With --in-reply-to one can send top-level cover and use it as base for sending other b4 managed branches.

This patch introduces 'b4 --in-reply-to IDENTIFIER' for adding 'In-Reply-To:' header to cover-letter message.