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>

git-rebase--interactive.sh

@@ -533,11 +533,10 @@ do_next () {
 	test -s "$todo" && return
 
 	comment_for_reflog finish &&
-	shortonto=$(git rev-parse --short $onto) &&
 	newhead=$(git rev-parse HEAD) &&
 	case $head_name in
 	refs/*)
-		message="$GIT_REFLOG_ACTION: $head_name onto $shortonto" &&
+		message="$GIT_REFLOG_ACTION: $head_name onto $onto" &&
 		git update-ref -m "$message" $head_name $newhead $orig_head &&
 		git symbolic-ref \
 		  -m "$GIT_REFLOG_ACTION: returning to $head_name" \