Fix stray comma in leaf-changes-only mode.

Fix stray comma in leaf-changes-only mode.

This change replaces the "\n," sometimes seen in output (such as for
the PR25128 test cases) with a correctly indented "and".

	* src/abg-reporter-priv.cc (represent): Don't try to follow
        output of indented pretty representation with a comma, just
        emit "and" unconditionally; remove unnecessary intermediate
        ostringstream.
	* tests/data/Makefile.am: Add new test case files.
	* tests/data/test-abidiff-exit/test-no-stray-comma-*: New test
        cases.
        * tests/data/test-diff-suppr/test46-PR25128-report-?.txt:
        Replace unindented comma with indented "and".
	* tests/test-abidiff-exit.cc: Add no-stray-comma test case.

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