Revision 1575525

Fix and regression test for issue #4427, "export external files".

Exports from working copies were omitting file externals due
to a logic bug in the externals handling of svn_client_export5().

* subversion/libsvn_client/export.c
  (export_info_baton): Add 'exporting_external' flag.
  (export_node): Don't skip file externals if 'exporting_external' flag is set.
  (do_export): Wrapper for logic of svn_client_export5(), with an additional
   'exporting_external' parameter which is passed to exporting_node() via
   the export_info_baton. Set the flag if recursing into do_export() to
   export an external.
  (svn_client_export5): Re-implement as wrapper of do_export().

* subversion/tests/cmdline/export_tests.py
  (export_file_external, test_list): New regression test which passes
   with the above change and fails without it.