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>

test-parse-options.c

@@ -67,7 +67,7 @@ int main(int argc, const char **argv)
 		OPT_GROUP("Standard options"),
 		OPT__ABBREV(&abbrev),
 		OPT__VERBOSE(&verbose, "be verbose"),
-		OPT__DRY_RUN(&dry_run),
+		OPT__DRY_RUN(&dry_run, "dry run"),
 		OPT__QUIET(&quiet),
 		OPT_END(),
 	};