t642[23]: be more flexible for add/add conflicts involving pair renames

t642[23]: be more flexible for add/add conflicts involving pair renames

Much like the last commit accepted 'add/add' and 'rename/add'
interchangably, we also want to do the same for 'add/add' and
'rename/rename'.  This also allows us to avoid the ambiguity in meaning
with 'rename/rename' (is it two separate files renamed to the same
location, or one file renamed on both sides but differently)?

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