sequencer: add update-ref command

sequencer: add update-ref command

Add the boilerplate for an "update-ref" command in the sequencer. This
connects to the current no-op do_update_ref() which will be filled in
after more connections are created.

The syntax in the todo list will be "update-ref <ref-name>" to signal
that we should store the current commit as the value for updating
<ref-name> at the end of the rebase.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>