backport.pl: Fix parsing logsummary of entries with a branch name in the revisions line, such as 1.8.x-svnsync-serf-memory.

* tools/dist/backport.pl
  (parse_entry): Consume the first line if we found a branch name in it.  The
    revnum-parsing loop will run on the subsequent line (and probably find
    nothing).

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1515337 13f79535-47bb-0310-9956-ffa450edef68