test-subrepo-svn: fix strange svn behaviour on windows

I got a full path when pusing the external like:

  Sending        C:/foo/bar/test-subrepo-svn/sub/t/s/alpha

instead of the relative path

tests/test-subrepo-svn

@@ -63,7 +63,8 b' echo'
 echo % change file in svn and hg, commit
 echo a >> a
 echo alpha >> s/alpha
-hg commit -m 'Message!' | sed "$filterexternal"
+hg commit -m 'Message!' | sed "$filterexternal" \
+    | sed 's:Sending.*s/alpha:Sending        s/alpha:g'
 hg debugsub | sed "$filterpath"
 
 echo