Display the proper URL for a deletion when prompting for the log message.

The existing code assumes that the relpaths passed to it are relative to the
repository root.  Rather, the code in delete_urls_multi_repos() groups URLs by
repository root and then condenses the URLs to find the longest possible session
parent.  This means if you issued the command:
  svn rm https://svn.apache.org/repos/asf/subversion/branches/1.8.x-r1579588

You'll end up seeing the following in your editor:
  D https://svn.apache.org/repos/asf/1.8.x-r1579588

* subversion/libsvn_client/delete.c
  (single_repos_delete): Rename repos_root to base_uri since we don't need the
    root but rather the session parent.
  (delete_urls_multi_repos): Remove repos_root variable that is no longer used,
    pass the base_uri to single_repos_delete().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1591123 13f79535-47bb-0310-9956-ffa450edef68