backport.pl: Minor improvements to sanitize_branch() helper.

* tools/dist/backport.pl
  (sanitize_branch): Add a docstring.  Restore support for some input formats
    that r1541813 accidentally removed (presumably because they weren't
    documented ☺).

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1568246 13f79535-47bb-0310-9956-ffa450edef68