backport.pl: Be more flexible about indentation.

The initial indentation before the * will determine how deep
everything else needs to be.  So if for instance someone puts
the * in the first column, it'll work as long as the relative
indents are all correct.

* tools/dist/backport.pl
  (parse_entry): calculate the relative indentation and strip it.
  (backport_main): asterisk can be in any position with or without
    leading spaces to start a backport entry.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1567523 13f79535-47bb-0310-9956-ffa450edef68