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

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>