t4058: explore duplicate tree entry handling in a bit more detail

t4058: explore duplicate tree entry handling in a bit more detail

While creating the last commit, I found a number of other cases where
git would segfault when faced with trees that have duplicate entries.
None of these segfaults are in the diffcore-rename code (they all occur
in cache-tree and unpack-trees).  Further, to my knowledge, no one has
ever been adversely affected by these bugs, and given that it has been
15 years and folks have fixed a few other issues with historical
duplicate entries (as noted in the last commit), I am not sure we will
ever run into anyone having problems with these.  So I am not sure these
are worth fixing, but it doesn't hurt to at least document these
failures in the same test file that is concerned with duplicate tree
entries.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>