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>

userdiff.h

@@ -11,9 +11,13 @@ struct userdiff_funcname {
 	int cflags;
 };
 
+struct external_diff {
+	char *cmd;
+};
+
 struct userdiff_driver {
 	const char *name;
-	char *external;
+	struct external_diff external;
 	char *algorithm;
 	int binary;
 	struct userdiff_funcname funcname;