rebase -i: use full onto sha1 in reflog

rebase -i: use full onto sha1 in reflog

'git rebase' uses the full onto sha1 for the reflog message whereas 'git
rebase -i' uses the short sha1. This is not only inconsistent, but can
lead to problems when the reflog is inspected at a later time at which
that abbreviation may have become ambiguous.

Make 'rebase -i' use the full onto sha1, as well.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>