rebase: replace antiquated sed invocation

Use the modern form of printing a commit subject instead of piping
the output of rev-list to sed.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -85,10 +85,8 @@ continue_merge () {
 			printf "Already applied: %0${prec}d " $msgnum
 		fi
 	fi
-	if test -z "$GIT_QUIET"
-	then
-		git rev-list --pretty=oneline -1 "$cmt" | sed -e 's/^[^ ]* //'
-	fi
+	test -z "$GIT_QUIET" &&
+	git log --format=%s -1 "$cmt"
 
 	prev_head=`git rev-parse HEAD^0`
 	# save the resulting commit so we can read-tree on it later