b4: prep --edit-cover emits strange message about "Already Ran"

[Dave Marquardt <davemarq@linux.ibm.com>]

I found the message here a little confusing, and had to go code
spelunking to figure out what it might mean and how I should answer the
Y/N question.

    $ b4 prep --edit-cover
    Waiting for Emacs...
    Invoking git-filter-repo to update the cover letter.
    The previous run is older than a day (/home/davemarq/linux/linux/.git/worktrees/ibmvfc-fpin-bis/filter-repo/already_ran already exists).
    See "Already Ran" section in the manual for more information.
    Treat this run as a continuation of filtering in the previous run (Y/N)? n
    New history written in 612.22 seconds...
    Completely finished after 612.53 seconds.
    Cover letter updated.

I first searched the b4 manual, and found nothing about "Already Ran".
Eventually, on reading the b4 code in ez.py, I found this was from the
git-filter-repo package and I should check its manual. I found the code.
There's no way to opt out of this check and message.

Would you be interested in updating the b4 manual with a note about
this? I eventually figured out I should answer with N. Perhaps an update
to the --edit-cover text in "Prep command flags" in the manual? I'll
provide it if you're interested.

-Dave