sequencer: write useful reflog message for fast-forward

The following command

  $ git cherry-pick --ff b8bb3f

writes the following uninformative message to the reflog

  cherry-pick

Improve it to

  cherry-pick: fast-forward

Avoid hard-coding "cherry-pick" in fast_forward_to(), so the sequencer
is generic enough to support future actions.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>