t6423: rename file within directory that other side renamed

t6423: rename file within directory that other side renamed

Add a new testcase where one side of history renames:
   olddir/ -> newdir/
and the other side of history renames:
   olddir/a -> olddir/alpha

When using merge.directoryRenames=true, it seems logical to expect the
file to end up at newdir/alpha.  Unfortunately, both merge-recursive and
merge-ort currently see this as a rename/rename conflict:

   olddir/a -> newdir/a
vs.
   olddir/a -> newdir/alpha

Suggesting that there's some extra logic we probably want to add
somewhere to allow this case to run without triggering a conflict.  For
now simply document this known issue.

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