Resolve regression, where for externals status absolute paths where provided
even though the caller requested relative paths.

(Our test suite currently ignores these lines. This patch is a preparation
 for fixing that)

* subversion/libsvn_client/client.h
  (svn_client__do_external_status): Provide information on how to show paths
    as relative. Rename pool to scratch_pool

* subversion/libsvn_client/externals.c
  (svn_client__do_external_status): Calculate relative paths if requested.

* subversion/libsvn_client/status.c
  (svn_client_status5): Update caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1434750 13f79535-47bb-0310-9956-ffa450edef68