contrib/git-svn: fix a harmless warning on rebuild (with old repos)

It's only for repositories that were imported with very early
versions of git-svn.  Unfortunately, some of those repos are out
in the wild already, so fix this warning.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

contrib/git-svn/git-svn.perl

@@ -155,7 +155,7 @@ sub rebuild {
 		# if we merged or otherwise started elsewhere, this is
 		# how we break out of it
 		next if (defined $SVN_UUID && ($uuid ne $SVN_UUID));
-		next if (defined $SVN_URL && ($url ne $SVN_URL));
+		next if (defined $SVN_URL && defined $url && ($url ne $SVN_URL));
 
 		print "r$rev = $c\n";
 		unless (defined $latest) {