tests: make tests for convert with svn portable svn 1.6.x (at least, 1.6.12 or 1.6.17) might display empty lines, even though svn 1.9.x (at least, 1.9.3) doesn't. To make tests for convert with svn portable, this patch adds "|(^$)" regexp to egrep in filter_svn_output. To avoid similar future issue, this patch adds "|(^$)" regexp to all filter_svn_output (and adjusts test-subrepo-svn.t), even though only test-convert-svn-source.t fails with svn 1.6.x, AFAIK.

--- a/tests/test-convert-svn-source.t	Tue Mar 15 14:10:46 2016 -0700
+++ b/tests/test-convert-svn-source.t	Tue Mar 15 00:14:53 2016 +0900
@@ -1,7 +1,7 @@
 #require svn svn-bindings
 
   $ filter_svn_output () {
-  >     egrep -v 'Committing|Updating' | sed -e 's/done$//' || true
+  >     egrep -v 'Committing|Updating|(^$)' | sed -e 's/done$//' || true
   > }
 
   $ cat >> $HGRCPATH <<EOF