diff: run external diff helper with shell

This is mostly to make it more consistent with the rest of
git, which uses the shell to exec helpers.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -2275,7 +2275,7 @@ static void run_external_diff(const char *pgm,
 	}
 	*arg = NULL;
 	fflush(NULL);
-	retval = run_command_v_opt(spawn_arg, 0);
+	retval = run_command_v_opt(spawn_arg, RUN_USING_SHELL);
 	remove_tempfile();
 	if (retval) {
 		fprintf(stderr, "external diff died, stopping at %s.\n", name);