sequencer: handle rebase-merges for "onto" message

sequencer: handle rebase-merges for "onto" message

In order to work correctly, git-rebase --rebase-merges needs to make
initial todo list with unique labels.

Those unique labels is being handled by employing a hashmap and
appending an unique number if any duplicate is found.

But, we forget that beside those labels for side branches,
we also have a special label `onto' for our so-called new-base.

In a special case that any of those labels for side branches named
`onto', git will run into trouble.

Correct it.

Signed-off-by: Doan Tran Cong Danh <congdanhqx@gmail.com>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>