diff.c

Shelving: Implement 'svn shelf-diff', issue SVN-4745.

This is a first pass. It drives only the file methods in the diff processor
API, not the open- and close-directory methods, and not reporting changes on
directories.

* subversion/include/svn_client.h
(svn_client_shelf_export_patch): Deprecate.

* subversion/include/private/svn_client_private.h
(svn_client__get_diff_writer_unidiff): New.
(svn_client__shelf_diff): New.

* subversion/libsvn_client/diff.c
(svn_client__get_diff_writer_unidiff): New.

* subversion/libsvn_client/shelf.c
(file_changed, file_deleted, file_added): New.
(diff_baton_t, diff_visitor): Drive a diff-processor from shelved changes.
(svn_client__shelf_diff): New.
(svn_client_shelf_export_patch): Delete the body; it is deprecated and did
nothing before.

* subversion/svn/shelf-cmd.c
(shelf_diff): First obtain a diff writer. Then call the shelf-diff API,
driving that writer.

* subversion/tests/cmdline/shelf_tests.py
(shelf_diff_simple): New test.
(test_list): Run it.