sequencer (rebase -i): implement the 'noop' command

sequencer (rebase -i): implement the 'noop' command

The 'noop' command is probably the most boring of all rebase -i commands
to support in the sequencer.

Which makes it an excellent candidate for this first stab to add support
for rebase -i's commands to the sequencer.

For the moment, let's also treat empty lines and commented-out lines as
'noop'; We will refine that handling later in this patch series.

To make it easier to identify "classes" of todo_commands (such as:
determine whether a command is pick-like, i.e. handles a single commit),
let's enforce a certain order of said commands.

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