backport.pl: guard against ambiguous --accept specifications.

* tools/dist/backport.pl
  (parse_entry): Detect multiple --accept specifications, warn about them,
    and ignore the conflicting values (so that the merge, later on, will
    conflict).

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1509649 13f79535-47bb-0310-9956-ffa450edef68