userdiff.c: use designated initializers for "struct userdiff_driver"

userdiff.c: use designated initializers for "struct userdiff_driver"

Change the "struct userdiff_driver" assignmentns to use designated
initializers, but let's keep the PATTERNS() and IPATTERN() convenience
macros to avoid churn, but have them defined in terms of designated
initializers.

For the "driver_true" and "driver_false" let's have the compiler
implicitly initialize most of the fields, but let's leave a redundant
".binary = 0" for "driver_true" to make it obvious that it's the
opposite of the the ".binary = 1" for "driver_false".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>