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

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

This regression was introduced by commit 0aa958d (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: Markus Heidelberg <redacted>
Signed-off-by: Junio C Hamano <redacted>