rebase: don't invoke the pager for each commit summary

This regression was introduced by commit 0aa958d4 (rebase: replace
antiquated sed invocation, 2010-01-24), which changed the invocation of
"git rev-list | sed" to "git log".

It can be reproduced by something like this:
$ git rebase -s recursive origin/master
Signed-off-by: NMarkus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -86,7 +86,7 @@ continue_merge () {
 		fi
 	fi
 	test -z "$GIT_QUIET" &&
-	git log --format=%s -1 "$cmt"
+	GIT_PAGER='' 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