[b4] make 'b4 mbox' reorder headers so piping to 'git imap-send' works

[Philippe Blain <levraiphilippeblain@gmail.com>]

Hi,

I'm using 'b4 mbox -o- <url> | git imap-send' to quickly import
threads to my email account when I see an interesting thread that
I'd like to reply to (I contribute to Git but I'm not subscribed to the
Git mailing list).

This only works if the emails in the threads have the 'From', 'Date'
and 'Subject' headers in exactly that order, because that order is
hard-coded in 'git imap-send' [1]. I wrote a small script [2] that
just reads the headers and reorders them accordingly but I'd like to
just use 'b4 mbox' instead as it has more features. However I discovered
that such reordering is not done by b4. For example, this fails as 'git imap-send'
reports 'nothing to send':

b4 mbox -o- https://lore.kernel.org/git/op.0vvmwohypvqxoc@damia/T/#u | git imap-send

It would be nice if b4 would take care of the reordering.

Cheers,

Philippe.

[1] https://git-scm.com/docs/git-imap-send#_description
[2] https://gist.github.com/phil-blain/d350e91959efa6e7afce60e74bf7e4a8