Как насчет небольшого сценария bash?
>> git show-ref | while read sha1 ref; do echo $(git log --pretty=format:%ai -1 $sha1) $ref ; done | sort
2010-10-23 15:30:57 +0200 refs/remotes/mambo/mambo
2010-10-23 15:30:57 +0200 refs/remotes/mambo/master
2010-10-29 22:38:23 +0200 refs/heads/master
2010-10-29 22:38:23 +0200 refs/remotes/nas/HEAD
2010-10-29 22:38:23 +0200 refs/remotes/nas/master