t/lib-rebase: prepare for testing `git rebase --rebase-merges`

t/lib-rebase: prepare for testing `git rebase --rebase-merges`

The format of the todo list is quite a bit different in the
`--rebase-merges` mode; Let's prepare the fake editor to handle those
todo lists properly, too.

The original idea was that we keep the original command unless
overridden, and because the original todo lists only had `pick` lines
anyway, we could be sloppy and "override" the command by the same
command (i.e. use the sed replacement pattern "pick" instead of "&").

This actually would not have worked with `fixup` and `squash` commands,
but it would appear that we never tried to use the fake editor with
`--autosquash`.

However, in the next commit we want to use the fake editor in
conjunction with `--rebase-merges`, so let's use the correct sed
replacement pattern.

Technically, it is not necessary to take care of the `fakesha` thing
(where we reuse the sed replacement pattern to craft a new todo
command), at least for now, as the only user of that thing overrides the
`action` anyway. Nevertheless, for completeness' sake, we do take care
of it.

Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>