Sorry for posting in already closed issue, but it looks for me, that it hasn't been solved. I updated to the latest 2.5-beta-2 subversion plugin (and Jenkins v 1.583) but still have "-1" issue. Strange that only one external repository is affected.
As you can see in the polling log, it's trigger to build a job cause plugin reads -1 revision.
Started on Oct 2, 2014 10:10:00 AM
Received SCM poll call on master for ProjectX on Oct 2, 2014 10:10:00 AM
https://host1/svn/repo1/tags/latest is at revision 136
https://host2/svn/repo2/trunk is at revision 44
https://host2/svn/repo2/trunk/tests is at revision 974
(changed from -1)
https://host1/repo3/trunk/path is at revision 83
Done. Took 1.7 sec
All listed repositories are svn:externals.