rebase -i: fix reflog message

rebase -i: fix reflog message

When rebase -i was converted to C a bug was introduced into the code
that creates the reflog message. Instead of saying
rebase -i (finish): <head-name> onto <onto>
it says
rebase -i (finish): <head-name> onto <orig-head><onto>
as the strbuf is not reset between reading the value of <orig-head>
and <onto>.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Acked-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>