t6019: test file dropped in -s ours merge

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: Kevin Bracey <kevin@bracey.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6019-rev-list-ancestry-path.sh

@@ -16,6 +16,9 @@ test_description='--ancestry-path'
 #
 #  F...I                 == F G H I
 #  --ancestry-path F...I == F H I
+#
+#  G..M -- G.t                 == [nothing - was dropped in "-s ours" merge L]
+#  --ancestry-path G..M -- G.t == H J L
 
 . ./test-lib.sh
 
@@ -89,6 +92,22 @@ test_expect_success 'rev-list --ancestry-path F...I' '
 	test_cmp expect actual
 '
 
+# G.t is dropped in an "-s ours" merge
+test_expect_success 'rev-list G..M -- G.t' '
+	>expect &&
+	git rev-list --format=%s G..M -- G.t |
+	sed -e "/^commit /d" >actual &&
+	test_cmp expect actual
+'
+
+test_expect_failure 'rev-list --ancestry-path G..M -- G.t' '
+	for c in H J L; do echo $c; done >expect &&
+	git rev-list --ancestry-path --format=%s G..M -- G.t |
+	sed -e "/^commit /d" |
+	sort >actual &&
+	test_cmp expect actual
+'
+
 #   b---bc
 #  / \ /
 # a   X