In an attempt to make cmpilato fix the rest of issue #3590 before 1.8, add
a foreign repository copy implementation to libsvn_client.

* subversion/include/private/svn_client_private.h
  (svn_client__copy_foreign): Declare new api. Still private.

* subversion/libsvn_client/copy_foreign.c
  (new file): Implement an editor that uses the new svn_wc_add_from_disk2()
    to perform a foreign repository copy.

* subversion/tests/libsvn_client/client-test.c
  (test_foreign_repos_copy): New test.
  (test_funcs): Add test_foreign_repos_copy.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1437141 13f79535-47bb-0310-9956-ffa450edef68