rebase --root: fix amending root commit messages

rebase --root: fix amending root commit messages

The code path that triggered that "BUG" really does not want to run
without an explicit commit message. In the case where we want to amend a
commit message, we have an *implicit* commit message, though: the one of
the commit to amend. Therefore, this code path should not even be
entered.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>