Enable custom keyword expansion for Ev2 export, fix repository root
URL expansion.

* subversion/libsvn_client/export.c
  (struct edit_baton): Add repos_root_url member.
  (add_file, export_file): Set repos_root_url correctly.
  (add_file_ev2): Use svn_subst_build_keywords3.
  (export_directory): Move svn_ra_get_repos_root2 call ...
  (svn_client_export5): ... here.

* subversion/tests/cmdline/export_tests.py
  (export_custom_keywords): New test.
  (test_list): Add new test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1466839 13f79535-47bb-0310-9956-ffa450edef68