Correctly pad large revision numbers in 'svnadmin verify --keep-going' summary.

* subversion/svnadmin/svnadmin.c
  (subcommand_verify): Like 'svn blame', keep the default revision number
   padding width at 6 columns, and increase padding for revisions > 1000000.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1546150 13f79535-47bb-0310-9956-ffa450edef68