parseopt: wrap rev-parse --parseopt usage for eval consumption

9c7304e (print the usage string on stdout instead of stderr,
2010-05-17) broke rev-parse --parseopt: when run with -h, the usage
notice on stdout ended up in the shell eval.

Wrap the usage in a cat <<\EOF ... EOF block when printing to stdout.
I do not expect any usage lines to ever start with EOF so this
shouldn't be an undue burden.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

parse-options.h

@@ -37,6 +37,7 @@ enum parse_opt_option_flags {
 	PARSE_OPT_NODASH = 32,
 	PARSE_OPT_LITERAL_ARGHELP = 64,
 	PARSE_OPT_NEGHELP = 128,
+	PARSE_OPT_SHELL_EVAL = 256
 };
 
 struct option;