Add a --dump-diff-tree to abidiff for debugging purposes

Add a --dump-diff-tree to abidiff for debugging purposes

I have felt the need to emit a textual representation of the diff
nodes tree maintained by the comparison engine for changed functions
and variables.  This patch adds that functionality.

	* include/abg-comparison.h (enum visiting_kind): Add new
	DO_NOT_MARK_VISITED_NODES_AS_TRAVERSED enumerator.
	(diff_context::{default_output_stream, error_output_stream,
	dump_diff_tree}): Declare new accessors.
	(diff::end_traversing): Take a new boolean flag.
	(print_diff_tree): Add new overload for diff_sptr.
	* src/abg-comparison.cc
	(diff_context::priv::{default_output_stream_,
	error_output_stream_, dump_diff_tree_}): New data members.
	(priv::priv): Initialize them.
	(diff_context::{default_output_stream_, error_output_stream_,
	dump_diff_tree, dump_diff_tree}): Define new accessors.
	(diff::end_traversing): Take a new flag that control whether or
	not to mark the current diff node as having been traversed.
	(diff::traverse): Take in account the visiting kind carried by the
	visitor to determine if the visited node should be marked as being
	traversed.
	(corpus_diff::priv::maybe_dump_diff_tree): Define new member
	function.
	(corpus_diff::report): Call it.
	(diff_node_printer::visit): Pretty print the diff node just once.
	(print_diff_tree): Define a new overload for diff_sptr.
	* tools/abidiff.cc (options::dump_diff_tree): New data member.
	(options::options): Initialize it.
	(display_usage): Add a help string for the new --dump-diff-tree
	command line switch.
	(parse_command_line): Parse the new --dump-diff-tree command line
	switch.
	(set_diff_context_from_opts): Set the diff context according to
	the --dump-diff-tree presence.
	* doc/manuals/abidiff.rst: Add a bullet point for the new
	--dump-diff-tree command line switch.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>