warnings buildbots.

Found

r1821860

r1821860 | julianfoad | 2018-01-22 13:04:31 +0000 (Mon, 22 Jan 2018)

Don't cast away 'const'.

Found by: warnings buildbots.

* subversion/include/svn_client.h,
  subversion/libsvn_client/shelf.c
  (svn_client_shelf_get_patch_abspath): Let the output be const.

* subversion/svn/shelf-cmd.c
  (show_diffstat): Update caller.