svnpoller: Don't fail when revisions contain changes outside our prefix.

When getting the logs, revisions that don't affect the SVNPoller.svnurl path
are already being filtered out by svn.  Since the poller doesn't even consider
them, I would expect it not to consider unrelated paths that come with the logs
and just filter them out.

Fixes #385.
svnpoller: Replace an assert with a real test.
Fix review comment.
Merge pull request #621 from tomprince/bug385

 svnpoller: Don't fail when revisions contain changes outside our prefix.