git-svn: skip mergeinfo handling with --no-follow-parent

For repositories without parent following enabled, finding
git parents through svn:mergeinfo or svk::parents can be
expensive and pointless.

Reported-by: Александр Овчинников <proff@proff.email>
http://mid.gmane.org/4094761466408188@web24o.yandex.ru

Signed-off-by: Eric Wong <e@80x24.org>