[BUG] rebase -i with empty commits + exec

[Stefan Beller <sbeller@google.com>]

Currently I am working on a longer series, for which I decided
to keep track of progress in an empty commit. This empty commit
is in the middle of the series (to divide the commits into two sets,
the foundation that I consider stable and the later parts that are not
as stable for my development, they contain things that may be useful)

Then I invoked "git rebase -i <base> -x make" to see
in which shape the series is.

The editor opened proposing the following instruction sheet,
which in my opinion is buggy:

    pick 1234 some commit
    exec make
    pick 2345 another commit
    exec make
    pick 3456 third commit
    # pick 4567 empty commit
    exec make
    pick 5678  yet another commit
    exec make

I think the lines of the empty commit and the following exec should
be swapped, because that exec should work on the third commit.
Maybe we'd want to see another commented exec:

    pick 1234 some commit
    exec make
    pick 2345 another commit
    exec make
    pick 3456 third commit
    exec make
    # pick 4567 empty commit
    # exec make <- unsure about this line
    pick 5678  yet another commit
    exec make

Thoughts?