b4: use new trailers in 'b4 send --resend'

[Luca Ceresoli <luca.ceresoli@bootlin.com>]

Hello,

first of all: thanks for b4, I love it when sending my patches!

I found a little issue when resending a patch series with unmodified
content, but with new trailers received after the initial sending.

This are the events involved:

 1. send a patch series (b4 send)
 2. receive some Reviewed-by, but patch not applied and no changes
    needed
 3. update trailers (b4 trailers -u)
 4. resend (b4 send --resend)

At step 4 I would expect b4 to prepare a [PATCH RESEND] series with
the new trailers added. Instead b4 resends the exact same series,
without any new trailers.

According to my experience in contributing to the Linux kernel, having
the trailers added when resending is the primary goal when resending a
series that needs no other changes, so I think it should be done by b4
automatically.

Best regards,
Luca

-- 
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com