Respect --exclude-common-ancestry for linear ancestries.

* bzrlib/tests/test_log.py:
(TestLogExcludeAncestry.test_merge_sorted_simple_revnos_exclude_ancestry):
There are two different code paths that exhibit bug #575631.

* bzrlib/log.py:
(_calc_view_revisions, _generate_flat_revisions): Propagate
exclude_common_ancestry into more code paths.
(_linear_view_revisions): Don't yield start_rev_id when we want to
exclude the common ancestry.