sequencer: don't abbreviate a command if it doesn't have a short form

sequencer: don't abbreviate a command if it doesn't have a short form

When the sequencer is requested to abbreviate commands, it will replace
those that do not have a short form (eg. `noop') by a comment mark.
`noop' serves no purpose, except when fast-forwarding (ie. by running
`git rebase').  Removing it will break this command when
`rebase.abbreviateCommands' is set to true.

Teach todo_list_to_strbuf() to check if a command has an actual
short form, and to ignore it if not.

Signed-off-by: Alban Gruin <redacted>
Signed-off-by: Junio C Hamano <redacted>