t4058: add more tests and documentation for duplicate tree entry handling

Commit 4d6be03b95 ("diffcore-rename: avoid processing duplicate
destinations", 2015-02-26) added t4058 to demonstrate that a workaround
it added to avoid double frees (namely to just turn off rename detection
when trees had duplicate entries) would indeed avoid segfaults.  The
tests, though, give the impression that the expected diffs are "correct"
when in reality they are just "don't segfault, and do something
semi-reasonable under the circumstances".  Add some notes to make this
clearer.

Also, commit 25d5ea410f ("[PATCH] Redo rename/copy detection logic.",
2005-05-24) added a similar workaround to avoid segfaults, but for
rename_src rather than rename_dst.  I do not see any tests in the
testsuite to cover the collision detection of entries limited to the
source side, so add a couple.

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

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