t6422, t6426: be more flexible for add/add conflicts involving renames

t6422, t6426: be more flexible for add/add conflicts involving renames

merge-recursive treats an add/add conflict where one of the adds came
from a rename as a separate 'rename/add' type of conflict.  However, if
there is not content conflict after the content merge(s), then the file
is not considered to be conflicted.  That suggests the conflict type is
really just add/add.  Other merge engines might choose to print messages
to the console that just refer to these as add/add conflicts; accept
both types of output.

Note: it could help to notify users if the three-way content merge of
the rename had content conflicts, because when we then go to two-way
merge THAT with the conflicting add we can get nested conflict markers.
merge-recursive, unfortunately, doesn't do that, but other merge engines
could.

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