backport.pl: Clean up merge() parser for "Branch:" entries.


* tools/dist/backport.pl
  (merge): Simplify the pattern used to detect the header.  Implement an
    escaping noted as a TODO in a comment.  Note that the first disjunct
    of the incumbent code was dead code since the ( ) ? weren't escaped
    correctly (so were taken to be literal, due to the \V; see ':h /\V').
  (parse_entry): Don't allow a "Branches:" header (plural), because
    handle_entry()/merge() don't know about plurality of branches in one entry.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507686 13f79535-47bb-0310-9956-ffa450edef68
* tools/dist/backport.pl
  (merge): Whitespace changes only.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507687 13f79535-47bb-0310-9956-ffa450edef68