bisect: add tests to document expected behaviour in presence of broken trees.

If the repo is broken, we expect bisect to fail.

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6030-bisect-porcelain.sh

@@ -581,5 +581,53 @@ test_expect_success 'erroring out when using bad path parameters' '
 '
 
 #
+# This creates a broken branch which cannot be checked out because
+# the tree created has been deleted.
 #
+# H1-H2-H3-H4-H5-H6-H7  <--other
+#            \
+#             S5-S6'-S7'-S8'-S9  <--broken
+#
+# Commits marked with ' have a missing tree.
+#
+test_expect_success 'broken branch creation' '
+	git bisect reset &&
+	git checkout -b broken $HASH4 &&
+	git tag BROKEN_HASH4 $HASH4 &&
+	add_line_into_file "5(broken): first line on a broken branch" hello2 &&
+	git tag BROKEN_HASH5 &&
+	mkdir missing &&
+	:> missing/MISSING &&
+	git add missing/MISSING &&
+	git commit -m "6(broken): Added file that will be deleted"
+	git tag BROKEN_HASH6 &&
+	add_line_into_file "7(broken): second line on a broken branch" hello2 &&
+	git tag BROKEN_HASH7 &&
+	add_line_into_file "8(broken): third line on a broken branch" hello2 &&
+	git tag BROKEN_HASH8 &&
+	git rm missing/MISSING &&
+	git commit -m "9(broken): Remove missing file"
+	git tag BROKEN_HASH9 &&
+	rm .git/objects/39/f7e61a724187ab767d2e08442d9b6b9dab587d
+'
+
+echo "" > expected.ok
+cat > expected.missing-tree.default <<EOF
+fatal: unable to read tree 39f7e61a724187ab767d2e08442d9b6b9dab587d
+EOF
+
+test_expect_success 'bisect fails if tree is broken on start commit' '
+	git bisect reset &&
+	test_must_fail git bisect start BROKEN_HASH7 BROKEN_HASH4 2>error.txt &&
+	test_cmp expected.missing-tree.default error.txt
+'
+
+test_expect_success 'bisect fails if tree is broken on trial commit' '
+	git bisect reset &&
+	test_must_fail git bisect start BROKEN_HASH9 BROKEN_HASH4 2>error.txt &&
+	git reset --hard broken &&
+	git checkout broken &&
+	test_cmp expected.missing-tree.default error.txt
+'
+
 test_done