`b4 prep` "not a prep-managed branch" error with changed Git email

Hi Konstantin, tools,

Igor (in Cc) contacted me since he was having a blocking issue with
`b4 prep` to send a v2 of a patch series. He was consistently getting
the following error:

  CRITICAL: This is not a prep-managed branch.

I don't use the `prep` subcommand myself, but I tried it and couldn't
reproduce it, so I suggested contacting you with a reproducer, and in
order to do so, I suggested getting the Git log `--fuller` of the
branch. Then I noticed he had a different committer than the
Signed-off-by -- I recalled he was changing his email recently in
another patch to `MAINTAINERS`.

And, indeed, if I changed my Git `user.email` I could reproduce the
issue. In Igor's side, the actual trigger was that for v2 of that
patch series he was adding an entry to the `.mailmap`, i.e. the
effective Git user email that appears in `git log` is what counts.

Is it intended? If so, could perhaps `b4` suggest that may be the
cause in the error message? (or even check that is indeed the issue)

I hope this helps, and thanks!

Cheers,
Miguel