Clarify two backward incompatible repository options.

Clarify two backward incompatible repository options.

It was unclear if the backward compatible features were disabled
or the configuration variables that controls them were set to
false by default from the description.  Obviously we meant the
former, but the problem was made worse by the fact that one
configuration variable breaks compatibility when set to true and
the other one breaks it when set to false.

Signed-off-by: Junio C Hamano <redacted>