Disallow methods other than GET/HEAD for the parentpath list.

Fixes the segfault for `svn ls http://svn.example.com` when SVN is handling
the server root and SVNListParentPath is on.

* subversion/mod_dav_svn/repos.c
  (get_resource): Return an error when we try to get a parentpath list
    resource and the method isn't GET.

Found by: lgo


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1557320 13f79535-47bb-0310-9956-ffa450edef68