Fix corpus_diff::has_net_changes for --leaf-changes-only mode

Fix corpus_diff::has_net_changes for --leaf-changes-only mode

This function was not aware of --leaf-changes-only mode.

    - Stats counters for changed variables and types have
      different names in the different modes.
    - Net leaf type changes were not included in leaf mode.

For some inputs, this resulted in abidiff producing an empty report
but returning a non-zero exit status in --leaf-changes-only mode.

For other inputs the combination of both issues still resulted in the
correct return code. This included the following test-abidiff-exit
test cases:

    - test-leaf-peeling
    - test-leaf2
    - test-no-stray-comma

This patch makes corpus_diff::has_net_changes mirror emit_diff_stats,
modulo flags like --non-reachable-types which if absent can still
result in discrepancies between output and return code.

To achieve this in a more maintainable way, the patch introduces a new interface
reporter_base::diff_has_net_changes.  That interface is implemented by
all current reporters.  Each reporter focuses on its own
particularities to provide the required behavious. Then
corpus_diff:has_net_changes just has to invoke
reporter_base::diff_has_net_changes on the reporter that is currently
in used.

The tests below verify that the exit code is zero when all the changes
between the test files are suppressed.

	* include/abg-reporter.h ({reporter_base, default_reporter,
	leaf_reporter}::diff_has_net_changes): Add new virtual function.
	This breaks binary compatibility but should conserve source
	compatibility.
	* src/abg-default-reporter.cc
	(default_reporter::diff_has_net_changes): Define new member
	function.
	* src/abg-leaf-reporter.cc (leaf_reporter::diff_has_net_changes):
	Likewise.
	* src/abg-comparison.cc (corpus_diff::has_net_changes): Invoke
	reporter_base::diff_has_net_changes on the current reporter,
	rather than trying to handle all the different kinds of reporters
	here.
	(corpus_diff::priv::apply_filters_and_compute_diff_stats): Add a
	TODO to possibly delegate the implementation of this function to
	the reporters.
	* tests/data/Makefile.am: Add new test case files.
	* tests/data/test-abidiff-exit/test-net-change-report0.txt:
	Normal mode, nothing suppressed.
	* tests/data/test-abidiff-exit/test-net-change-report1.txt:
	Normal mode, everything suppressed.
	* tests/data/test-abidiff-exit/test-net-change-report2.txt:
	Leaf mode, nothing suppressed.
	* tests/data/test-abidiff-exit/test-net-change-report3.txt:
	Leaf mode, everything suppressions.
	* tests/data/test-abidiff-exit/test-net-change-v0.c: Test file
	* tests/data/test-abidiff-exit/test-net-change-v0.o: Test file
	* tests/data/test-abidiff-exit/test-net-change-v1.c: Test file
	* tests/data/test-abidiff-exit/test-net-change-v1.o: Test file
	* tests/data/test-abidiff-exit/test-net-change.abignore: This
	suppresses changes for all variables, functions and types in
	the test files, except for the 'victim' function.
	* tests/test-abidiff-exit.cc: Run new test cases.

Reviewed-by: Matthias Maennich <maennich@google.com>
Signed-off-by: Giuliano Procida <gprocida@google.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>