Rename diff::length() into diff::has_changes()

Rename diff::length() into diff::has_changes()

Since it turned out that the length of the changes carried by a diff
node has never been used in the algorithms of the comparison engine,
the diff::length() feels wrong.  What we want is rather a name like
diff::has_changes() so this is what this patch does.

	* include/abg-comparison.h (*::has_changes): Rename the ::length()
	method of all the diff types that inherit the diff class into
	this, in the class declarations.
	* src/abg-comparison.cc (*::has_changes): Do the same as in the
	declarations, in the definitions.
	(diff::to_be_reported, distinct_diff::has_local_changes)
	(distinct_diff::report, distinct_diff::, array_diff::has_changes)
	(reference_diff::has_changes, qualified_type_diff::has_changes)
	(enum_diff::has_changes, translation_unit_diff::has_changes)
	(suppression_categorization_visitor::visit_end)
	(redundancy_marking_visitor::visit_begin): Adjust.
	* tests/test-diff-dwarf.cc (main): Adjust.
	* tools/abidiff.cc (main): Likewise.

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