rev-list: detect broken root trees

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>