"git rev-list --first-parent --boundary $commit^..$commit" segfaults on a
merge commit since 8d2dfc4 (process_{tree,blob}: show objects without
buffering, 2009-04-10), as it tried to dereference a commit that was
discarded as UNINTERESTING without being parsed (hence lacking "tree").
Signed-off-by: Junio C Hamano <gitster@pobox.com>
