sequencer: fix --allow-empty-message behavior, make it smarter

sequencer: fix --allow-empty-message behavior, make it smarter

In commit b00bf1c9a8dd ("git-rebase: make --allow-empty-message the
default", 2018-06-27), several arguments were given for transplanting
empty commits without halting and asking the user for confirmation on
each commit.  These arguments were incomplete because the logic clearly
assumed the only cases under consideration were transplanting of commits
with empty messages (see the comment about "There are two sources for
commits with empty messages).  It didn't discuss or even consider
rewords, squashes, etc. where the user is explicitly asked for a new
commit message and provides an empty one.  (My bad, I totally should
have thought about that at the time, but just didn't.)

Rewords and squashes are significantly different, though, as described
by SZEDER:

    Let's suppose you start an interactive rebase, choose a commit to
    squash, save the instruction sheet, rebase fires up your editor, and
    then you notice that you mistakenly chose the wrong commit to
    squash.  What do you do, how do you abort?

    Before [that commit] you could clear the commit message, exit the
    editor, and then rebase would say "Aborting commit due to empty
    commit message.", and you get to run 'git rebase --abort', and start
    over.

    But [since that commit, ...] saving the commit message as is would
    let rebase continue and create a bunch of unnecessary objects, and
    then you would have to use the reflog to return to the pre-rebase
    state.

Also, he states:

    The instructions in the commit message template, which is shown for
    'reword' and 'squash', too, still say...

    # Please enter the commit message for your changes. Lines starting
    # with '#' will be ignored, and an empty message aborts the commit.

These are sound arguments that when editing commit messages during a
sequencer operation, that if the commit message is empty then the
operation should halt and ask the user to correct.  The arguments in
commit b00bf1c9a8dd (referenced above) still apply when transplanting
previously created commits with empty commit messages, so the sequencer
should not halt for those.

Furthermore, all rationale so far applies equally for cherry-pick as for
rebase.  Therefore, make the code default to --allow-empty-message when
transplanting an existing commit, and to default to halting when the
user is asked to edit a commit message and provides an empty one -- for
both rebase and cherry-pick.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>