Rename ONE_FILESYSTEM to DISCOVERY_ACROSS_FILESYSTEM

Rename ONE_FILESYSTEM to DISCOVERY_ACROSS_FILESYSTEM

If a missing ONE_FILESYSTEM defaults to true, the only users who set this
variable set it to false to tell git not to limit the discovery to one
filesystem; there are too many negations in one sentence to make a simple
panda brain dizzy.

Use the variable GIT_DISCOVERY_ACROSS_FILESYSTEM that changes the
behaviour from the default "limit to one filesystem" to "cross the
boundary as I ask you to"; makes the semantics much more straight
forward.

Signed-off-by: Junio C Hamano <gitster@pobox.com>