replay: add an important FIXME comment about gpg signing

replay: add an important FIXME comment about gpg signing

We want to be able to handle signed commits in some way in the future,
but we are not ready to do it now. So for the time being let's just add
a FIXME comment to remind us about it.

These are different ways we could handle them:

  - in case of a cli user and if there was an interactive mode, we could
    perhaps ask if the user wants to sign again
  - we could add an option to just fail if there are signed commits

Co-authored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>