add description parameter to OPT__DRY_RUN

Allows better help text to be defined than "dry run".  Also make use
of the macro in places that already had a different description.  No
object code changes intended.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/remote.c

@@ -1200,7 +1200,7 @@ static int prune(int argc, const char **argv)
 {
 	int dry_run = 0, result = 0;
 	struct option options[] = {
-		OPT__DRY_RUN(&dry_run),
+		OPT__DRY_RUN(&dry_run, "dry run"),
 		OPT_END()
 	};