backport.pl: Tolerate malformed separator lines.

When the separator line between two entries is missing or contains whitespace
(r'^\s+$' instead of r'^$'), svn-role would skip the entries and report parse
errors.  Instead, teach it to correct those separator lines and proceed.

Correction of separator lines is done in a dedicated commit.

* tools/dist/backport.pl
  (renormalize_STATUS): New helper.
  (backport_main): Add --renormalize flag, which calls renormalize_STATUS().
  (): Document that svn-role will use --renormalize.
  (merge): Tweak the STATUS changes to make renormalization more often a noop.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1568597 13f79535-47bb-0310-9956-ffa450edef68