b4 change: "send --prefixes" is now "prep --set-prefixes"

b4 change: "send --prefixes" is now "prep --set-prefixes"

[Konstantin Ryabitsev]

Hi, all:

The previous way of dealing with extra subject line prefixes was to append
them during the "b4 send" stage:

    b4 send --prefixes RFC

I realized that some subsystems require more permanent prefixes, such as
[PATCH netdev] or [PATCH bpf], and it was too easy to forget to add them each
time the "send" command was invoked. For this reason, I'm making the prefixes
"sticky" across revisions.

For example, if you want to send an RFC patch series to netdev:

    b4 prep --set-prefixes RFC netdev

To remove the RFC prefix once you're ready for it:

    b4 prep --set-prefixes netdev

To remove all prefixes:

    b4 prep --set-prefixes ''

In addition to the "--set-prefixes" command, these extra prefixes can also be
added to the cover letter subject line (and edited via "b4 prep --edit-cover"):

    [RFC netdev] Cover letter subject line

    Cover letter body.
    ---
    Changes in v2:
    ...

This change will be in the 0.11 version of b4, which will probably be released
later today or tomorrow.

-K