Partial fix for Issue #443: post-commit hook script (error) output lost
This is step 6 : Pass the post-commit error from the server to the client
over all three protocols.

* subversion/libsvn_subr/constructors.c
(svn_commit_info_dup): Modified to copy the post_commit_err member.

* subversion/mod_dav_svn/merge.c
(global): included svn_xml.h.
(dav_svn__merge_response): Modified to use the new post-commit
error message parameter.

* subversion/mod_dav_svn/dav_svn.h
(dav_svn__merge_response): Add new parameter - post_commit_err.

* subversion/mod_dav_svn/version.c
(dav_svn_merge): Modified to extract the post-commit error message.

* subversion/clients/cmdline/util.c
(svn_cl__print_commit_info): At last, we can print out the post-commit
error messages! yoohoo!

* subversion/tests/clients/cmdline/commit_tests.py
(post_commit_hook_test): New function to test post-commit hook's error
output.
(test_list): Added post_commit_hook_test to list of tests.

* subversion/libsvn_repos/hooks.c
(svn_repos__hooks_post_commit): Pass TRUE for the read_errstream
parameter of run_hook_cmd.

* subversion/libsvn_ra_dav/commit.c
(commit_close_edit): Modified call to svn_ra_dav__merge_activity to
pass the post-commit error message.

* subversion/libsvn_ra_svn/client.c
(ra_svn_end_commit): Modified to read the extra post-commit error
message sent out by the server.

* subversion/libsvn_ra_svn/protocol
(commit_info): Added the optional parameter for post-commit error.

* subversion/libsvn_ra_dav/merge.c
(merge_elements): Added new member for post-commit error processing.
(merge_ctx_t): Added new member for post-commit error.
(validate_element): Added test for the ELEM_post_commit_err.
(end_element): Added post-commit error handling.
(svn_ra_dav__merge_activity): Added the post_commit_err parameter
and its handling.

* subversion/libsvn_ra_dav/ra_dav.h
(enum): Added a new member ELEM_post_commit_err.
(svn_ra_dav__merge_activity): Added new parameter for post-commit error
message.

* subversion/libsvn_ra_dav/commit.c
(commit_close): Include post_commit_err in call to svn_ra_dav__merge_activity.

* subversion/svnserve/serve.c
(commit_callback_baton_t): Add a new member to hold the post-commit
error message.
(commit_done): Modified to copy the over the post-commit error message
to the callback baton.
(commit): Write out the tuple with the post-commit error message.