backport.pl: renormalize: Permit whitespace lines inside entries.

Suggested by: breser

* tools/dist/backport.pl
  (renormalize_STATUS): As above.
    While here, extend commentary.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1569243 13f79535-47bb-0310-9956-ffa450edef68