More fixes related to externals handling

That MR contains two commits with the following fixes:

• If an external definition targets an URL whose starts with 'svn+ssh://@', the checkout or export operation is blocking due to SSH password prompt triggered by such URL.

  So ensure to automatically set the requested password to 'anonymous' to avoid blocking when attempting to export such external.

• Previous implementation of is_recursive_external detected some false positive cases which were leading to hash mismatches when checking revisions.

This fixes the full loading of https://svn.code.sf.net/p/contrap/code.