The URL's to the TFS repository browser (diff.aspx en cs.aspx) are changed in TFS 2013. Next is an overview of the new URL's.

current webaccess URL:
http://<TFS-SERVER>:8080/tfs/Main/cs.aspx?cs=<CHANGESET-NR>
TFS 2013 URL:
http://<TFS-SERVER>:8080/tfs/Main/_versionControl/changeset/<CHANGESET-NR>

current diff URL:
http://<TFS-SERVER>:8080/tfs/Main/diff.aspx?opath=<FILE1>&ocs=<CHANGESET-FILE1>&mpath=<FILE2>&mcs=<CHANGESET-FILE2>
TFS 2013 URL:
http:/<TFS-SERVER>:8080/tfs/Main/_versionControl/changeset/<CHANGESET-FILE2>#path=<FILE>&_a=compare
(Notice that you don't need to give the previous changeset number)