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.

tests/test-convert-hg-svn.t

@@ -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 <<EOF >> $HGRCPATH

tests/test-convert-svn-source.t

@@ -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

tests/test-subrepo-svn.t

@@ -8,7 +8,7
 #endif
 
   $ filter_svn_output () {
-  >     egrep -v 'Committing|Transmitting|Updating' || true
+  >     egrep -v 'Committing|Transmitting|Updating|(^$)' || true
   > }
 
 create subversion repo
@@ -97,10 +97,8 change file in svn and hg, commit
   committing subrepository s
   Sending*s/alpha (glob)
   Committed revision 3.
-  
   Fetching external item into '*s/externals'* (glob)
   External at revision 1.
-  
   At revision 3.
   $ hg debugsub
   path s