In preparation for upcoming TREESAME work, check the result for G.t,
which is dropped in "-s ours" merge L. The default rev-list is empty, as
expected - it follows the first parent path where it never existed.

Unfortunately, --ancestry-path is also empty. Merges H J and L are all
TREESAME to 1 parent, so are treated as TREESAME and not shown. This is
clearly undesirable in the case of merge L, which dropped our G.t by
taking the non-ancestry-path version. Document this as a known failure,
and expect "H J L", the 3 merges along the path that had to chose G.t
versions.
Signed-off-by: NKevin Bracey <kevin@bracey.fi>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>