sequencer: drop "warning:" when stopping for edit

sequencer: drop "warning:" when stopping for edit

Since the conversion from shell to C in 56dc3ab04 (sequencer
(rebase -i): implement the 'edit' command, 2017-01-02),
stopping at an "edit" instruction went from:

  $ git rebase -i
  Stopped at 6ce6b914a... odb_pack_keep(): stop generating keepfile name
  You can amend the commit now, with
    [...more instructions...]

to:

  $ git rebase -i
  warning: stopped at 6ce6b914a... odb_pack_keep(): stop generating keepfile name
  You can amend the commit now, with
    [...more instructions...]

The "warning" implies that it's something unexpected, but
it's not. Let's switch back to the original message.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>