git-svn: platform auth providers are working only on 1.6.15 or newer

git-svn: platform auth providers are working only on 1.6.15 or newer

Matthijs Kooijman reports that the cut-off point 082afee (git-svn:
use platform specific auth providers, 2012-04-26) set at 1.6.12 to
use this feature safely was incorrect, and it is 1.6.15 instead:

    http://svn.apache.org/repos/asf/subversion/trunk/CHANGES
    Version 1.6.15
       * improve some swig parameter mapping (r984565, r1035745)

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Eric Wong <normalperson@yhbt.net>