When we parse userdiff config, we generally assume that

  diff.name.key

will affect the "key" value of the "name" driver. However,
without confirming that the key is a valid userdiff key, we
may accidentally conflict with the ancient "diff.color.*"
namespace. The current code is careful not to even create a
driver struct if we do not see a key that is known by the
diff-driver code.

However, this carefulness is unnecessary; the default driver
with no keys set behaves exactly the same as having no
driver at all. We can simply set up the driver struct as
soon as we see we have a config key that looks like a
driver. This makes the code a bit more readable.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>