btrbk-mail: Lines with >>> are parsed as quotes by email clients

I'm using btrbk 0.27.1 wrapped inside the supplied `btrbk-mail` script, which sends me an email about success or failure. Unfortunately, the output of btrbk contains consecutive lines starting with `>>>` which is intepreted as email quotes by email cliens (Thunderbird in my case), making the email hard to read.

I would like to propose to use other characters instead. Maybe `»»»`?

Alternatively maybe a leading space would prevent the formatting? Or maybe there is an email header that can prevent formatting?