fetchsvn: Add ignoreExternals attribute.

There are some SVN repositories out there which don't have revision information
tied to externals. By using ignoreExternals, fetchsvn won't fetch these
externals anymore, so the fetch won't fail with a checksum mismatch, should
there be some changes in some of those external repositories.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>