new: b4 send --reflect

[Konstantin Ryabitsev <konstantin@linuxfoundation.org>]

Hi, all:

Final feature before 0.11 (hopefully) -- there is a new test mode for b4 send
that will only email the series to yourself instead of the actual destinations
listed in "To" or "Cc" headers.

This should be particularly useful to anyone using the web submission
endpoint, because this will allow them to see any post-processing that the
remote will do to their patches (such as rewriting From: for DMARC purposes).

If you're using b4 from git, I recommend that you get the latest master so it
can see some testing before the 0.11 release later in the week. 
The main notable new features in 0.11 as compared to 0.10.y are:

- b4 prep --compare-to N: shows a range-diff between your current prep branch
  and the previously submitted revision N

- b4 ty: can be configured to always send mail without requiring the -S switch
  (via setting b4.ty-send-mail=yes)

- b4 send will no longer send the entire series to the addresses found in
  individual commit Cc: trailers; such addresses will only receive the cover
  letter and the patches where they are explicitly Cc'd

- you can now set extra [PATCH] prefixes permanently either via b4 prep
  --set-prefixes or via the cover letter subject line

- internal changes to ensure that we generate 7-bit-clean headers for all
  outgoing mail

- b4 send --reflect: prepares the patches but only sends them to yourself

If all goes well, I will release 0.11 either Thursday or Friday.

Best wishes,
Konstantin