unpack-trees: fix D/F conflict bugs in verify_absent

I came across a few bugs while investigating the changes I proposed in the
modify/delete conflict thread. The first two are quite obvious. The third I'm
not so sure about. I could not find a testcase where it matters. Junio, do you
recall the original intention of that code?

[PATCH 1/3] unpack-trees: handle failure in verify_absent
[PATCH 2/3] unpack-trees: fix path search bug in verify_absent
[PATCH 3/3] unpack-trees: remove redundant path search in verify_absent

 t/t1001-read-tree-m-2way.sh |   51 +++++++++++++++++++++++++++++++++++++++++++
 unpack-trees.c              |   37 +++++++++++++++----------------
 2 files changed, 69 insertions(+), 19 deletions(-)