On the 'move-tracking-2' branch: Add a comment about branch ids being arbitrary.
On the 'move-tracking-2' branch: Add a comment about branch ids being
arbitrary.

* subversion/include/private/svn_branch.h,
  (svn_branch_revision_root_get_branch_by_id,
   svn_branch_get_id): Add a comment and a cross-reference.

git-svn-id: 486ec3464084f62cfe8608152673c827b947faa2@1676580 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion