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>

git-svn.perl

@@ -5481,7 +5481,7 @@ ()
 
 	# earlier 1.6.x versions would segfault, and <= 1.5.x didn't have
 	# this function
-	if (::compare_svn_version('1.6.12') > 0) {
+	if (::compare_svn_version('1.6.15') >= 0) {
 		my $config = SVN::Core::config_get_config($config_dir);
 		my ($p, @a);
 		# config_get_config returns all config files from