backport.pl: Get rid of an ugly hack.

This also fixes a bug introduced in r1569068 (which see for details).

* tools/dist/backport.pl
  (parse_entry):
    Store the paragraph number (relative to the file) in the parsed entry.
  (merge):
    Use the paragraph number to delete the entry.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1569235 13f79535-47bb-0310-9956-ffa450edef68