rev-list: detect broken root trees

When the traversal machinery sees a commit without a root tree, it
assumes that the tree was part of a BOUNDARY commit, and quietly ignores
the tree. But it could also be caused by a commit whose root tree is
broken or missing.

Instead, let's die() when we see a NULL root tree. We can differentiate
it from the BOUNDARY case by seeing if the commit was actually parsed.
This covers that case, plus future-proofs us against any others where we
might try to show an unparsed commit.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t6102-rev-list-unexpected-objects.sh b/t/t6102-rev-list-unexpected-objects.sh
index 28ee1bc..28611c9 100755
--- a/t/t6102-rev-list-unexpected-objects.sh
+++ b/t/t6102-rev-list-unexpected-objects.sh
@@ -67,8 +67,10 @@
 	test_must_fail git rev-list --objects $broken_commit
 '
 
-test_expect_failure 'traverse unexpected non-tree root (seen)' '
-	test_must_fail git rev-list --objects $blob $broken_commit
+test_expect_success 'traverse unexpected non-tree root (seen)' '
+	test_must_fail git rev-list --objects $blob $broken_commit \
+		>output 2>&1 &&
+	test_i18ngrep "not a tree" output
 '
 
 test_expect_success 'setup unexpected non-commit tag' '