userdiff: add and use struct external_diff

userdiff: add and use struct external_diff

Wrap the string specifying the external diff command in a new struct to
simplify adding attributes, which the next patch will do.

Make sure external_diff() still returns NULL if neither the environment
variable GIT_EXTERNAL_DIFF nor the configuration option diff.external is
set, to continue allowing its use in a boolean context.

Use a designated initializer for the default builtin userdiff driver to
adjust to the type change of the second struct member.  Spelling out
only the non-zero members improves readability as a nice side-effect.

No functional change intended.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>