diff-cmd.c

Shelving: implement shelf diff summary mode.

"svn shelf-diff --summarize"

* subversion/include/private/svn_client_private.h,
subversion/libsvn_client/client.h,
subversion/libsvn_client/diff_summarize.c
(svn_client__get_diff_summarize_callbacks): Move from a private to a
semi-public header file. Remove 'const' from the output parameter.

* subversion/libsvn_client/diff.c
(svn_client_diff_summarize2,
svn_client_diff_summarize_peg2): Tweak to match that 'const' change.

* subversion/svn/cl.h,
subversion/svn/diff-cmd.c
(svn_cl__get_diff_summarize): Newly exposed API.
(svn_cl__diff): Use it.

* subversion/svn/shelf-cmd.c
(shelf_diff): Implement summary mode.

* subversion/svn/svn.c
(svn_cl__cmd_table): Add '--summarize' option to shelf-diff.

* subversion/tests/cmdline/shelf_tests.py
(shelf_diff_simple): Extend to test summary mode.

* tools/client-side/bash_completion
(_svn): Add '--summarize' option to shelf-diff.