sequencer: ignore HEAD ref under --update-refs

sequencer: ignore HEAD ref under --update-refs

When using the 'git rebase -i --update-refs' option, the todo list is
populated with 'update-ref' commands for all tip refs in the history
that is being rebased. Refs that are checked out by some worktree are
instead added as a comment to warn the user that they will not be
updated.

Until now, this included the HEAD ref, which is being updated by the
rebase process itself, regardless of the --update-refs option. Remove
the comment in this case by ignoring any decorations that match the HEAD
ref.

Reported-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>